[PATCH v14 0/9] rust: add `Ownable` trait and `Owned` type

[PATCH v14 0/9] rust: add `Ownable` trait and `Owned` type

[Andreas Hindborg]

Add a new trait `Ownable` and type `Owned` for types that specify their
own way of performing allocation and destruction. This is useful for
types from the C side.

Add the trait `OwnableRefCounted` that allows conversion between
`ARef` and `Owned`. This is analogous to conversion between `Arc` and
`UniqueArc`.

Patch 2 is difficult to merge since it requires a change everywhere
`AlwaysRefCounted` is implemented prior to application. We had new
implementations of this trait for the last few releases, so we are
probably going to have some when these patches are applied. Sorry for
that. I would ask maintainers of all affected areas to ACK these patches
as soon as possible.

This series has been underway a very long time. The work was started
off by Asahi Lina, then picked up by Oliver Mangold and is now being
finalized by Andreas Hindborg. For series like this, it is difficult to
track what contributions came from what authors at what time. I'm going
to leave authorship of the patches with the person originally submitting
the patch. I will not track changes by each individual on each patch.
Instead, I will leave a link to the original patch in the commit message
of these multi-contributor patches.

As I cannot reach out to everyone to ask if they will sign off on
the changes I have made, I will remove their tags, as these require
approval.

With this approach, checkpatch.pl is complaining about missing
Signed-off-by from patch author. I am not sure if we can keep it like
this, if I need to change the author, or if I need to hunt down the
original author to get an ACK for keeping the Signed-off-by tag?

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
Changes in v14:
- Rebase on v6.19-rc7.
- Rewrite cover letter.
- Update documentation and safety comments based on v13 feedback.
- Update commit messages.
- Reorder implementation blocks in owned.rs.
- Update example in owned.rs to use try operator rather than `expect`.
- Reformat use statements.
- Add patch: rust: page: convert to `Ownable`.
- Add patch: rust: implement `ForeignOwnable` for `Owned`.
- Add patch: rust: page: add `from_raw()`.
- Link to v13: https://lore.kernel.org/r/20251117-unique-ref-v13-0-b5b243df1250@pm.me

Changes in v13:
- Rebase onto v6.18-rc1 (Andreas's work).
- Documentation and style fixes contributed by Andreas
- Link to v12: https://lore.kernel.org/r/20251001-unique-ref-v12-0-fa5c31f0c0c4@pm.me

Changes in v12:
-
- Rebase onto v6.17-rc1 (Andreas's work).
- moved kernel/types/ownable.rs to kernel/owned.rs
- Drop OwnableMut, make DerefMut depend on Unpin instead. I understood
  ML discussion as that being okay, but probably needs further scrunity.
- Lots of more documentation changes suggested by reviewers.
- Usage example for Ownable/Owned.
- Link to v11: https://lore.kernel.org/r/20250618-unique-ref-v11-0-49eadcdc0aa6@pm.me

Changes in v11:
- Rework of documentation. I tried to honor all requests for changes "in
  spirit" plus some clearifications and corrections of my own.
- Dropping `SimpleOwnedRefCounted` by request from Alice, as it creates a
  potentially problematic blanket implementation (which a derive macro that
  could be created later would not have).
- Dropping Miguel's "kbuild: provide `RUSTC_HAS_DO_NOT_RECOMMEND` symbol"
  patch, as it is not needed anymore after dropping `SimpleOwnedRefCounted`.
  (I can add it again, if it is considered useful anyway).
- Link to v10: https://lore.kernel.org/r/20250502-unique-ref-v10-0-25de64c0307f@pm.me

Changes in v10:
- Moved kernel/ownable.rs to kernel/types/ownable.rs
- Fixes in documentation / comments as suggested by Andreas Hindborg
- Added Reviewed-by comment for Andreas Hindborg
- Fix rustfmt of pid_namespace.rs
- Link to v9: https://lore.kernel.org/r/20250325-unique-ref-v9-0-e91618c1de26@pm.me

Changes in v9:
- Rebase onto v6.14-rc7
- Move Ownable/OwnedRefCounted/Ownable, etc., into separate module
- Documentation fixes to Ownable/OwnableMut/OwnableRefCounted
- Add missing SAFETY documentation to ARef example
- Link to v8: https://lore.kernel.org/r/20250313-unique-ref-v8-0-3082ffc67a31@pm.me

Changes in v8:
- Fix Co-developed-by and Suggested-by tags as suggested by Miguel and Boqun
- Some small documentation fixes in Owned/Ownable patch
- removing redundant trait constraint on DerefMut for Owned as suggested by Boqun Feng
- make SimpleOwnedRefCounted no longer implement RefCounted as suggested by Boqun Feng
- documentation for RefCounted as suggested by Boqun Feng
- Link to v7: https://lore.kernel.org/r/20250310-unique-ref-v7-0-4caddb78aa05@pm.me

Changes in v7:
- Squash patch to make Owned::from_raw/into_raw public into parent
- Added Signed-off-by to other people's commits
- Link to v6: https://lore.kernel.org/r/20250310-unique-ref-v6-0-1ff53558617e@pm.me

Changes in v6:
- Changed comments/formatting as suggested by Miguel Ojeda
- Included and used new config flag RUSTC_HAS_DO_NOT_RECOMMEND,
  thus no changes to types.rs will be needed when the attribute
  becomes available.
- Fixed commit message for Owned patch.
- Link to v5: https://lore.kernel.org/r/20250307-unique-ref-v5-0-bffeb633277e@pm.me

Changes in v5:
- Rebase the whole thing on top of the Ownable/Owned traits by Asahi Lina.
- Rename AlwaysRefCounted to RefCounted and make AlwaysRefCounted a
  marker trait instead to allow to obtain an ARef<T> from an &T,
  which (as Alice pointed out) is unsound when combined with UniqueRef/Owned.
- Change the Trait design and naming to implement this feature,
  UniqueRef/UniqueRefCounted is dropped in favor of Ownable/Owned and
  OwnableRefCounted is used to provide the functions to convert
  between Owned and ARef.
- Link to v4: https://lore.kernel.org/r/20250305-unique-ref-v4-1-a8fdef7b1c2c@pm.me

Changes in v4:
- Just a minor change in naming by request from Andreas Hindborg,
  try_shared_to_unique() -> try_from_shared(),
  unique_to_shared() -> into_shared(),
  which is more in line with standard Rust naming conventions.
- Link to v3: https://lore.kernel.org/r/Z8Wuud2UQX6Yukyr@mango

---
Andreas Hindborg (4):
      rust: aref: update formatting of use statements
      rust: page: update formatting of `use` statements
      rust: implement `ForeignOwnable` for `Owned`
      rust: page: add `from_raw()`

Asahi Lina (2):
      rust: types: Add Ownable/Owned types
      rust: page: convert to `Ownable`

Oliver Mangold (3):
      rust: rename `AlwaysRefCounted` to `RefCounted`.
      rust: Add missing SAFETY documentation for `ARef` example
      rust: Add `OwnableRefCounted`

 rust/kernel/auxiliary.rs        |   7 +-
 rust/kernel/block/mq/request.rs |  15 +-
 rust/kernel/cred.rs             |  13 +-
 rust/kernel/device.rs           |  10 +-
 rust/kernel/device/property.rs  |   7 +-
 rust/kernel/drm/device.rs       |  10 +-
 rust/kernel/drm/gem/mod.rs      |   8 +-
 rust/kernel/fs/file.rs          |  16 +-
 rust/kernel/i2c.rs              |  16 +-
 rust/kernel/lib.rs              |   1 +
 rust/kernel/mm.rs               |  15 +-
 rust/kernel/mm/mmput_async.rs   |   9 +-
 rust/kernel/opp.rs              |  10 +-
 rust/kernel/owned.rs            | 366 ++++++++++++++++++++++++++++++++++++++++
 rust/kernel/page.rs             |  57 +++++--
 rust/kernel/pci.rs              |  10 +-
 rust/kernel/pid_namespace.rs    |  12 +-
 rust/kernel/platform.rs         |   7 +-
 rust/kernel/sync/aref.rs        |  80 ++++++---
 rust/kernel/task.rs             |  10 +-
 rust/kernel/types.rs            |  13 +-
 21 files changed, 612 insertions(+), 80 deletions(-)
---
base-commit: 63804fed149a6750ffd28610c5c1c98cce6bd377
change-id: 20250305-unique-ref-29fcd675f9e9

Best regards,
-- 
Andreas Hindborg <a.hindborg@kernel.org>

[PATCH v14 1/9] rust: types: Add Ownable/Owned types

[Andreas Hindborg]

From: Asahi Lina <lina+kernel@asahilina.net>

By analogy to `AlwaysRefCounted` and `ARef`, an `Ownable` type is a
(typically C FFI) type that *may* be owned by Rust, but need not be. Unlike
`AlwaysRefCounted`, this mechanism expects the reference to be unique
within Rust, and does not allow cloning.

Conceptually, this is similar to a `KBox<T>`, except that it delegates
resource management to the `T` instead of using a generic allocator.

This change is a derived work based on work by Asahi Lina
<lina+kernel@asahilina.net> [1] and Oliver Mangold <oliver.mangold@pm.me>.

Link: https://lore.kernel.org/rust-for-linux/20250202-rust-page-v1-1-e3170d7fe55e@asahilina.net/ [1]
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/lib.rs       |   1 +
 rust/kernel/owned.rs     | 196 +++++++++++++++++++++++++++++++++++++++++++++++
 rust/kernel/sync/aref.rs |   5 ++
 rust/kernel/types.rs     |  11 ++-
 4 files changed, 212 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index f812cf1200428..96a3fadc3377a 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -119,6 +119,7 @@
 pub mod of;
 #[cfg(CONFIG_PM_OPP)]
 pub mod opp;
+pub mod owned;
 pub mod page;
 #[cfg(CONFIG_PCI)]
 pub mod pci;
diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
new file mode 100644
index 0000000000000..fe30580331df9
--- /dev/null
+++ b/rust/kernel/owned.rs
@@ -0,0 +1,196 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Unique owned pointer types for objects with custom drop logic.
+//!
+//! These pointer types are useful for C-allocated objects which by API-contract
+//! are owned by Rust, but need to be freed through the C API.
+
+use core::{
+    mem::ManuallyDrop,
+    ops::{
+        Deref,
+        DerefMut, //
+    },
+    pin::Pin,
+    ptr::NonNull, //
+};
+
+/// Types that specify their own way of performing allocation and destruction. Typically, this trait
+/// is implemented on types from the C side.
+///
+/// Implementing this trait allows types to be referenced via the [`Owned<Self>`] pointer type. This
+/// is useful when it is desirable to tie the lifetime of the reference to an owned object, rather
+/// than pass around a bare reference. [`Ownable`] types can define custom drop logic that is
+/// executed when the owned reference [`Owned<Self>`] pointing to the object is dropped.
+///
+/// Note: The underlying object is not required to provide internal reference counting, because it
+/// represents a unique, owned reference. If reference counting (on the Rust side) is required,
+/// [`AlwaysRefCounted`](crate::types::AlwaysRefCounted) should be implemented.
+///
+/// # Safety
+///
+/// Implementers must ensure that the [`release()`](Self::release) function frees the underlying
+/// object in the correct way for a valid, owned object of this type.
+///
+/// # Examples
+///
+/// A minimal example implementation of [`Ownable`] and its usage with [`Owned`] looks like
+/// this:
+///
+/// ```
+/// # #![expect(clippy::disallowed_names)]
+/// # use core::cell::Cell;
+/// # use core::ptr::NonNull;
+/// # use kernel::sync::global_lock;
+/// # use kernel::alloc::{flags, kbox::KBox, AllocError};
+/// # use kernel::types::{Owned, Ownable};
+///
+/// // Let's count the allocations to see if freeing works.
+/// kernel::sync::global_lock! {
+///     // SAFETY: we call `init()` right below, before doing anything else.
+///     unsafe(uninit) static FOO_ALLOC_COUNT: Mutex<usize> = 0;
+/// }
+/// // SAFETY: We call `init()` only once, here.
+/// unsafe { FOO_ALLOC_COUNT.init() };
+///
+/// struct Foo;
+///
+/// impl Foo {
+///     fn new() -> Result<Owned<Self>> {
+///         // We are just using a `KBox` here to handle the actual allocation, as our `Foo` is
+///         // not actually a C-allocated object.
+///         let result = KBox::new(
+///             Foo {},
+///             flags::GFP_KERNEL,
+///         )?;
+///         let result = NonNull::new(KBox::into_raw(result))
+///             .expect("Raw pointer to newly allocation KBox is null, this should never happen.");
+///         // Count new allocation
+///         *FOO_ALLOC_COUNT.lock() += 1;
+///         // SAFETY: We just allocated the `Self`, thus it is valid and there cannot be any other
+///         // Rust references. Calling `into_raw()` makes us responsible for ownership and we won't
+///         // use the raw pointer anymore. Thus we can transfer ownership to the `Owned`.
+///         Ok(unsafe { Owned::from_raw(result) })
+///     }
+/// }
+///
+/// // SAFETY: The implementation of `release` in this trait implementation correctly frees the
+/// // owned `Foo`.
+/// unsafe impl Ownable for Foo {
+///     unsafe fn release(this: NonNull<Self>) {
+///         // SAFETY: The [`KBox<Self>`] is still alive. We can pass ownership to the [`KBox`], as
+///         // by requirement on calling this function, the `Self` will no longer be used by the
+///         // caller.
+///         drop(unsafe { KBox::from_raw(this.as_ptr()) });
+///         // Count released allocation
+///         *FOO_ALLOC_COUNT.lock() -= 1;
+///     }
+/// }
+///
+/// {
+///    let foo = Foo::new().expect("Failed to allocate a Foo. This shouldn't happen");
+///    assert!(*FOO_ALLOC_COUNT.lock() == 1);
+/// }
+/// // `foo` is out of scope now, so we expect no live allocations.
+/// assert!(*FOO_ALLOC_COUNT.lock() == 0);
+/// ```
+pub unsafe trait Ownable {
+    /// Releases the object.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that:
+    /// - `this` points to a valid `Self`.
+    /// - `*this` is no longer used after this call.
+    unsafe fn release(this: NonNull<Self>);
+}
+
+/// A mutable reference to an owned `T`.
+///
+/// The [`Ownable`] is automatically freed or released when an instance of [`Owned`] is
+/// dropped.
+///
+/// # Invariants
+///
+/// - The [`Owned<T>`] has exclusive access to the instance of `T`.
+/// - The instance of `T` will stay alive at least as long as the [`Owned<T>`] is alive.
+pub struct Owned<T: Ownable> {
+    ptr: NonNull<T>,
+}
+
+impl<T: Ownable> Owned<T> {
+    /// Creates a new instance of [`Owned`].
+    ///
+    /// This function takes over ownership of the underlying object.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that:
+    /// - `ptr` points to a valid instance of `T`.
+    /// - Ownership of the underlying `T` can be transferred to the `Self<T>` (i.e. operations
+    ///   which require ownership will be safe).
+    /// - An `Owned<T>` is a mutable reference to the underlying object. As such,
+    ///   the object must not be accessed (read or mutated) through any pointer
+    ///   other than the created `Owned<T>`. Opt-out is still possible similar to
+    ///   a mutable reference (e.g. by using [`Opaque`]).
+    ///
+    /// [`Opaque`]: kernel::types::Opaque
+    pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
+        // INVARIANT: By function safety requirement:
+        // - The resulting object has exclusive access to the `T` pointed to by `ptr`.
+        // - The `T` object pointed to by `ptr` is alive at least as long as the returned `Self`.
+        Self { ptr }
+    }
+
+    /// Consumes the [`Owned`], returning a raw pointer.
+    ///
+    /// This function does not drop the underlying `T`. When this function returns, ownership of the
+    /// underlying `T` is with the caller.
+    pub fn into_raw(me: Self) -> NonNull<T> {
+        ManuallyDrop::new(me).ptr
+    }
+
+    /// Get a pinned mutable reference to the data owned by this `Owned<T>`.
+    pub fn get_pin_mut(&mut self) -> Pin<&mut T> {
+        // SAFETY: The type invariants guarantee that the object is valid, and that we can safely
+        // return a mutable reference to it.
+        let unpinned = unsafe { self.ptr.as_mut() };
+
+        // SAFETY: We never hand out unpinned mutable references to the data in
+        // `Self`, unless the contained type is `Unpin`.
+        unsafe { Pin::new_unchecked(unpinned) }
+    }
+}
+
+// SAFETY: It is safe to send an [`Owned<T>`] to another thread when the underlying `T` is [`Send`],
+// because of the ownership invariant. Sending an [`Owned<T>`] is equivalent to sending the `T`.
+unsafe impl<T: Ownable + Send> Send for Owned<T> {}
+
+// SAFETY: It is safe to send [`&Owned<T>`] to another thread when the underlying `T` is [`Sync`],
+// because of the ownership invariant. Sending an [`&Owned<T>`] is equivalent to sending the `&T`.
+unsafe impl<T: Ownable + Sync> Sync for Owned<T> {}
+
+impl<T: Ownable> Deref for Owned<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The type invariants guarantee that the object is valid.
+        unsafe { self.ptr.as_ref() }
+    }
+}
+
+impl<T: Ownable + Unpin> DerefMut for Owned<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // SAFETY: The type invariants guarantee that the object is valid, and that we can safely
+        // return a mutable reference to it.
+        unsafe { self.ptr.as_mut() }
+    }
+}
+
+impl<T: Ownable> Drop for Owned<T> {
+    fn drop(&mut self) {
+        // SAFETY: The type invariants guarantee that the `Owned` owns the object we're about to
+        // release.
+        unsafe { T::release(self.ptr) };
+    }
+}
diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index 0d24a0432015d..e175aefe86151 100644
--- a/rust/kernel/sync/aref.rs
+++ b/rust/kernel/sync/aref.rs
@@ -29,6 +29,11 @@
 /// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
 /// instances of a type.
 ///
+/// Note: Implementing this trait allows types to be wrapped in an [`ARef<Self>`]. It requires an
+/// internal reference count and provides only shared references. If unique references are required
+/// [`Ownable`](crate::types::Ownable) should be implemented which allows types to be wrapped in an
+/// [`Owned<Self>`](crate::types::Owned).
+///
 /// # Safety
 ///
 /// Implementers must ensure that increments to the reference count keep the object alive in memory
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 9c5e7dbf16323..4aec7b699269a 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -11,7 +11,16 @@
 };
 use pin_init::{PinInit, Wrapper, Zeroable};
 
-pub use crate::sync::aref::{ARef, AlwaysRefCounted};
+pub use crate::{
+    owned::{
+        Ownable,
+        Owned, //
+    },
+    sync::aref::{
+        ARef,
+        AlwaysRefCounted, //
+    }, //
+};
 
 /// Used to transfer ownership to and from foreign (non-Rust) languages.
 ///

-- 
2.51.2

[PATCH v14 2/9] rust: rename `AlwaysRefCounted` to `RefCounted`.

[Andreas Hindborg]

From: Oliver Mangold <oliver.mangold@pm.me>

There are types where it may both be reference counted in some cases and
owned in others. In such cases, obtaining `ARef<T>` from `&T` would be
unsound as it allows creation of `ARef<T>` copy from `&Owned<T>`.

Therefore, we split `AlwaysRefCounted` into `RefCounted` (which `ARef<T>`
would require) and a marker trait to indicate that the type is always
reference counted (and not `Ownable`) so the `&T` -> `ARef<T>` conversion
is possible.

- Rename `AlwaysRefCounted` to `RefCounted`.
- Add a new unsafe trait `AlwaysRefCounted`.
- Implement the new trait `AlwaysRefCounted` for the newly renamed
  `RefCounted` implementations. This leaves functionality of existing
  implementers of `AlwaysRefCounted` intact.

Original patch by Oliver Mangold <oliver.mangold@pm.me> [1].

Link: https://lore.kernel.org/r/20251117-unique-ref-v13-2-b5b243df1250@pm.me [1]
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/auxiliary.rs        |  7 +++++-
 rust/kernel/block/mq/request.rs | 15 +++++++------
 rust/kernel/cred.rs             | 13 ++++++++++--
 rust/kernel/device.rs           | 10 ++++++---
 rust/kernel/device/property.rs  |  7 +++++-
 rust/kernel/drm/device.rs       | 10 ++++++---
 rust/kernel/drm/gem/mod.rs      |  8 ++++---
 rust/kernel/fs/file.rs          | 16 ++++++++++----
 rust/kernel/i2c.rs              | 16 +++++++++-----
 rust/kernel/mm.rs               | 15 +++++++++----
 rust/kernel/mm/mmput_async.rs   |  9 ++++++--
 rust/kernel/opp.rs              | 10 ++++++---
 rust/kernel/owned.rs            |  2 +-
 rust/kernel/pci.rs              | 10 ++++++++-
 rust/kernel/pid_namespace.rs    | 12 +++++++++--
 rust/kernel/platform.rs         |  7 +++++-
 rust/kernel/sync/aref.rs        | 47 ++++++++++++++++++++++++++---------------
 rust/kernel/task.rs             | 10 ++++++---
 rust/kernel/types.rs            |  3 ++-
 19 files changed, 164 insertions(+), 63 deletions(-)

diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
index be76f11aecb7e..c410dcfc7b6f7 100644
--- a/rust/kernel/auxiliary.rs
+++ b/rust/kernel/auxiliary.rs
@@ -11,6 +11,7 @@
     driver,
     error::{from_result, to_result, Result},
     prelude::*,
+    sync::aref::{AlwaysRefCounted, RefCounted},
     types::Opaque,
     ThisModule,
 };
@@ -283,7 +284,7 @@ unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for Device<Ctx>
 kernel::impl_device_context_into_aref!(Device);
 
 // SAFETY: Instances of `Device` are always reference-counted.
-unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
+unsafe impl RefCounted for Device {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::get_device(self.as_ref().as_raw()) };
@@ -302,6 +303,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
+// `&Device`.
+unsafe impl AlwaysRefCounted for Device {}
+
 impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
     fn as_ref(&self) -> &device::Device<Ctx> {
         // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
index ce3e30c81cb5e..cf013b9e2cacf 100644
--- a/rust/kernel/block/mq/request.rs
+++ b/rust/kernel/block/mq/request.rs
@@ -9,7 +9,7 @@
     block::mq::Operations,
     error::Result,
     sync::{
-        aref::{ARef, AlwaysRefCounted},
+        aref::{ARef, AlwaysRefCounted, RefCounted},
         atomic::Relaxed,
         Refcount,
     },
@@ -229,11 +229,10 @@ unsafe impl<T: Operations> Send for Request<T> {}
 // mutate `self` are internally synchronized`
 unsafe impl<T: Operations> Sync for Request<T> {}
 
-// SAFETY: All instances of `Request<T>` are reference counted. This
-// implementation of `AlwaysRefCounted` ensure that increments to the ref count
-// keeps the object alive in memory at least until a matching reference count
-// decrement is executed.
-unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {
+// SAFETY: All instances of `Request<T>` are reference counted. This implementation of `RefCounted`
+// ensure that increments to the ref count keeps the object alive in memory at least until a
+// matching reference count decrement is executed.
+unsafe impl<T: Operations> RefCounted for Request<T> {
     fn inc_ref(&self) {
         self.wrapper_ref().refcount().inc();
     }
@@ -255,3 +254,7 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) {
         }
     }
 }
+
+// SAFETY: We currently do not implement `Ownable`, thus it is okay to obtain an `ARef<Request>`
+// from a `&Request` (but this will change in the future).
+unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {}
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
index ffa156b9df377..20ef0144094be 100644
--- a/rust/kernel/cred.rs
+++ b/rust/kernel/cred.rs
@@ -8,7 +8,12 @@
 //!
 //! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
 
-use crate::{bindings, sync::aref::AlwaysRefCounted, task::Kuid, types::Opaque};
+use crate::{
+    bindings,
+    sync::aref::RefCounted,
+    task::Kuid,
+    types::{AlwaysRefCounted, Opaque},
+};
 
 /// Wraps the kernel's `struct cred`.
 ///
@@ -76,7 +81,7 @@ pub fn euid(&self) -> Kuid {
 }
 
 // SAFETY: The type invariants guarantee that `Credential` is always ref-counted.
-unsafe impl AlwaysRefCounted for Credential {
+unsafe impl RefCounted for Credential {
     #[inline]
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
@@ -90,3 +95,7 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) {
         unsafe { bindings::put_cred(obj.cast().as_ptr()) };
     }
 }
+
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Credential>` from a
+// `&Credential`.
+unsafe impl AlwaysRefCounted for Credential {}
diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index 031720bf5d8ca..e09dad5f9afea 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -7,8 +7,8 @@
 use crate::{
     bindings, fmt,
     prelude::*,
-    sync::aref::ARef,
-    types::{ForeignOwnable, Opaque},
+    sync::aref::{ARef, RefCounted},
+    types::{AlwaysRefCounted, ForeignOwnable, Opaque},
 };
 use core::{any::TypeId, marker::PhantomData, ptr};
 
@@ -492,7 +492,7 @@ pub fn fwnode(&self) -> Option<&property::FwNode> {
 kernel::impl_device_context_into_aref!(Device);
 
 // SAFETY: Instances of `Device` are always reference-counted.
-unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
+unsafe impl RefCounted for Device {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::get_device(self.as_raw()) };
@@ -504,6 +504,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
+// `&Device`.
+unsafe impl AlwaysRefCounted for Device {}
+
 // SAFETY: As by the type invariant `Device` can be sent to any thread.
 unsafe impl Send for Device {}
 
diff --git a/rust/kernel/device/property.rs b/rust/kernel/device/property.rs
index 3a332a8c53a9e..a8bb824ad0ec1 100644
--- a/rust/kernel/device/property.rs
+++ b/rust/kernel/device/property.rs
@@ -14,6 +14,7 @@
     fmt,
     prelude::*,
     str::{CStr, CString},
+    sync::aref::{AlwaysRefCounted, RefCounted},
     types::{ARef, Opaque},
 };
 
@@ -359,7 +360,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 }
 
 // SAFETY: Instances of `FwNode` are always reference-counted.
-unsafe impl crate::types::AlwaysRefCounted for FwNode {
+unsafe impl RefCounted for FwNode {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the
         // refcount is non-zero.
@@ -373,6 +374,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<FwNode>` from a
+// `&FwNode`.
+unsafe impl AlwaysRefCounted for FwNode {}
+
 enum Node<'a> {
     Borrowed(&'a FwNode),
     Owned(ARef<FwNode>),
diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 3ce8f62a00569..38ce7f389ed00 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -11,8 +11,8 @@
     error::from_err_ptr,
     error::Result,
     prelude::*,
-    sync::aref::{ARef, AlwaysRefCounted},
-    types::Opaque,
+    sync::aref::{AlwaysRefCounted, RefCounted},
+    types::{ARef, Opaque},
 };
 use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
 
@@ -198,7 +198,7 @@ fn deref(&self) -> &Self::Target {
 
 // SAFETY: DRM device objects are always reference counted and the get/put functions
 // satisfy the requirements.
-unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {
+unsafe impl<T: drm::Driver> RefCounted for Device<T> {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::drm_dev_get(self.as_raw()) };
@@ -213,6 +213,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
+// `&Device`.
+unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {}
+
 impl<T: drm::Driver> AsRef<device::Device> for Device<T> {
     fn as_ref(&self) -> &device::Device {
         // SAFETY: `bindings::drm_device::dev` is valid as long as the DRM device itself is valid,
diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index a7f682e95c018..ad6840a440165 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -10,8 +10,7 @@
     drm::driver::{AllocImpl, AllocOps},
     error::{to_result, Result},
     prelude::*,
-    sync::aref::{ARef, AlwaysRefCounted},
-    types::Opaque,
+    types::{ARef, AlwaysRefCounted, Opaque},
 };
 use core::{ops::Deref, ptr::NonNull};
 
@@ -253,7 +252,7 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
 }
 
 // SAFETY: Instances of `Object<T>` are always reference-counted.
-unsafe impl<T: DriverObject> crate::types::AlwaysRefCounted for Object<T> {
+unsafe impl<T: DriverObject> crate::types::RefCounted for Object<T> {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::drm_gem_object_get(self.as_raw()) };
@@ -267,6 +266,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
         unsafe { bindings::drm_gem_object_put(obj.as_raw()) }
     }
 }
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
+// `&Object`.
+unsafe impl<T: DriverObject> crate::types::AlwaysRefCounted for Object<T> {}
 
 impl<T: DriverObject> super::private::Sealed for Object<T> {}
 
diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index 23ee689bd2400..06e457d62a939 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -12,8 +12,8 @@
     cred::Credential,
     error::{code::*, to_result, Error, Result},
     fmt,
-    sync::aref::{ARef, AlwaysRefCounted},
-    types::{NotThreadSafe, Opaque},
+    sync::aref::RefCounted,
+    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
 };
 use core::ptr;
 
@@ -197,7 +197,7 @@ unsafe impl Sync for File {}
 
 // SAFETY: The type invariants guarantee that `File` is always ref-counted. This implementation
 // makes `ARef<File>` own a normal refcount.
-unsafe impl AlwaysRefCounted for File {
+unsafe impl RefCounted for File {
     #[inline]
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
@@ -212,6 +212,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<File>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<File>` from a
+// `&File`.
+unsafe impl AlwaysRefCounted for File {}
+
 /// Wraps the kernel's `struct file`. Not thread safe.
 ///
 /// This type represents a file that is not known to be safe to transfer across thread boundaries.
@@ -233,7 +237,7 @@ pub struct LocalFile {
 
 // SAFETY: The type invariants guarantee that `LocalFile` is always ref-counted. This implementation
 // makes `ARef<LocalFile>` own a normal refcount.
-unsafe impl AlwaysRefCounted for LocalFile {
+unsafe impl RefCounted for LocalFile {
     #[inline]
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
@@ -249,6 +253,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<LocalFile>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<LocalFile>` from a
+// `&LocalFile`.
+unsafe impl AlwaysRefCounted for LocalFile {}
+
 impl LocalFile {
     /// Constructs a new `struct file` wrapper from a file descriptor.
     ///
diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
index 39b0a9a207fda..b5e3c236a5c16 100644
--- a/rust/kernel/i2c.rs
+++ b/rust/kernel/i2c.rs
@@ -17,8 +17,10 @@
     of,
     prelude::*,
     types::{
+        ARef,
         AlwaysRefCounted,
-        Opaque, //
+        Opaque,
+        RefCounted, //
     }, //
 };
 
@@ -31,8 +33,6 @@
     }, //
 };
 
-use kernel::types::ARef;
-
 /// An I2C device id table.
 #[repr(transparent)]
 #[derive(Clone, Copy)]
@@ -416,7 +416,7 @@ pub fn get(index: i32) -> Result<ARef<Self>> {
 kernel::impl_device_context_into_aref!(I2cAdapter);
 
 // SAFETY: Instances of `I2cAdapter` are always reference-counted.
-unsafe impl crate::types::AlwaysRefCounted for I2cAdapter {
+unsafe impl crate::types::RefCounted for I2cAdapter {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::i2c_get_adapter(self.index()) };
@@ -427,6 +427,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
         unsafe { bindings::i2c_put_adapter(obj.as_ref().as_raw()) }
     }
 }
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from an
+// `&I2cAdapter`.
+unsafe impl AlwaysRefCounted for I2cAdapter {}
 
 /// The i2c board info representation
 ///
@@ -492,7 +495,7 @@ unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for I2cClient<C
 kernel::impl_device_context_into_aref!(I2cClient);
 
 // SAFETY: Instances of `I2cClient` are always reference-counted.
-unsafe impl AlwaysRefCounted for I2cClient {
+unsafe impl RefCounted for I2cClient {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::get_device(self.as_ref().as_raw()) };
@@ -503,6 +506,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
         unsafe { bindings::put_device(&raw mut (*obj.as_ref().as_raw()).dev) }
     }
 }
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from an
+// `&I2cClient`.
+unsafe impl AlwaysRefCounted for I2cClient {}
 
 impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for I2cClient<Ctx> {
     fn as_ref(&self) -> &device::Device<Ctx> {
diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
index 4764d7b68f2a7..dd9e3969e7206 100644
--- a/rust/kernel/mm.rs
+++ b/rust/kernel/mm.rs
@@ -13,8 +13,8 @@
 
 use crate::{
     bindings,
-    sync::aref::{ARef, AlwaysRefCounted},
-    types::{NotThreadSafe, Opaque},
+    sync::aref::RefCounted,
+    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
 };
 use core::{ops::Deref, ptr::NonNull};
 
@@ -55,7 +55,7 @@ unsafe impl Send for Mm {}
 unsafe impl Sync for Mm {}
 
 // SAFETY: By the type invariants, this type is always refcounted.
-unsafe impl AlwaysRefCounted for Mm {
+unsafe impl RefCounted for Mm {
     #[inline]
     fn inc_ref(&self) {
         // SAFETY: The pointer is valid since self is a reference.
@@ -69,6 +69,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Mm>` from a `&Mm`.
+unsafe impl AlwaysRefCounted for Mm {}
+
 /// A wrapper for the kernel's `struct mm_struct`.
 ///
 /// This type is like [`Mm`], but with non-zero `mm_users`. It can only be used when `mm_users` can
@@ -91,7 +94,7 @@ unsafe impl Send for MmWithUser {}
 unsafe impl Sync for MmWithUser {}
 
 // SAFETY: By the type invariants, this type is always refcounted.
-unsafe impl AlwaysRefCounted for MmWithUser {
+unsafe impl RefCounted for MmWithUser {
     #[inline]
     fn inc_ref(&self) {
         // SAFETY: The pointer is valid since self is a reference.
@@ -105,6 +108,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<MmWithUser>` from a
+// `&MmWithUser`.
+unsafe impl AlwaysRefCounted for MmWithUser {}
+
 // Make all `Mm` methods available on `MmWithUser`.
 impl Deref for MmWithUser {
     type Target = Mm;
diff --git a/rust/kernel/mm/mmput_async.rs b/rust/kernel/mm/mmput_async.rs
index b8d2f051225c7..aba4ce675c860 100644
--- a/rust/kernel/mm/mmput_async.rs
+++ b/rust/kernel/mm/mmput_async.rs
@@ -10,7 +10,8 @@
 use crate::{
     bindings,
     mm::MmWithUser,
-    sync::aref::{ARef, AlwaysRefCounted},
+    sync::aref::RefCounted,
+    types::{ARef, AlwaysRefCounted},
 };
 use core::{ops::Deref, ptr::NonNull};
 
@@ -34,7 +35,7 @@ unsafe impl Send for MmWithUserAsync {}
 unsafe impl Sync for MmWithUserAsync {}
 
 // SAFETY: By the type invariants, this type is always refcounted.
-unsafe impl AlwaysRefCounted for MmWithUserAsync {
+unsafe impl RefCounted for MmWithUserAsync {
     #[inline]
     fn inc_ref(&self) {
         // SAFETY: The pointer is valid since self is a reference.
@@ -48,6 +49,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<MmWithUserAsync>`
+// from a `&MmWithUserAsync`.
+unsafe impl AlwaysRefCounted for MmWithUserAsync {}
+
 // Make all `MmWithUser` methods available on `MmWithUserAsync`.
 impl Deref for MmWithUserAsync {
     type Target = MmWithUser;
diff --git a/rust/kernel/opp.rs b/rust/kernel/opp.rs
index a760fac287655..06fe2ca776a4f 100644
--- a/rust/kernel/opp.rs
+++ b/rust/kernel/opp.rs
@@ -16,8 +16,8 @@
     ffi::{c_char, c_ulong},
     prelude::*,
     str::CString,
-    sync::aref::{ARef, AlwaysRefCounted},
-    types::Opaque,
+    sync::aref::RefCounted,
+    types::{ARef, AlwaysRefCounted, Opaque},
 };
 
 #[cfg(CONFIG_CPU_FREQ)]
@@ -1041,7 +1041,7 @@ unsafe impl Send for OPP {}
 unsafe impl Sync for OPP {}
 
 /// SAFETY: The type invariants guarantee that [`OPP`] is always refcounted.
-unsafe impl AlwaysRefCounted for OPP {
+unsafe impl RefCounted for OPP {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
         unsafe { bindings::dev_pm_opp_get(self.0.get()) };
@@ -1053,6 +1053,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<OPP>` from an
+// `&OPP`.
+unsafe impl AlwaysRefCounted for OPP {}
+
 impl OPP {
     /// Creates an owned reference to a [`OPP`] from a valid pointer.
     ///
diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
index fe30580331df9..b02edda11fcf6 100644
--- a/rust/kernel/owned.rs
+++ b/rust/kernel/owned.rs
@@ -25,7 +25,7 @@
 ///
 /// Note: The underlying object is not required to provide internal reference counting, because it
 /// represents a unique, owned reference. If reference counting (on the Rust side) is required,
-/// [`AlwaysRefCounted`](crate::types::AlwaysRefCounted) should be implemented.
+/// [`RefCounted`](crate::types::RefCounted) should be implemented.
 ///
 /// # Safety
 ///
diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index bea76ca9c3da5..9ee8f2bc6db9e 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -19,6 +19,10 @@
     },
     prelude::*,
     str::CStr,
+    sync::aref::{
+        AlwaysRefCounted,
+        RefCounted, //
+    },
     types::Opaque,
     ThisModule, //
 };
@@ -467,7 +471,7 @@ unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for Device<Ctx>
 impl crate::dma::Device for Device<device::Core> {}
 
 // SAFETY: Instances of `Device` are always reference-counted.
-unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
+unsafe impl RefCounted for Device {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::pci_dev_get(self.as_raw()) };
@@ -479,6 +483,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
+// `&Device`.
+unsafe impl AlwaysRefCounted for Device {}
+
 impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
     fn as_ref(&self) -> &device::Device<Ctx> {
         // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
diff --git a/rust/kernel/pid_namespace.rs b/rust/kernel/pid_namespace.rs
index 979a9718f153d..4f6a94540e33d 100644
--- a/rust/kernel/pid_namespace.rs
+++ b/rust/kernel/pid_namespace.rs
@@ -7,7 +7,11 @@
 //! C header: [`include/linux/pid_namespace.h`](srctree/include/linux/pid_namespace.h) and
 //! [`include/linux/pid.h`](srctree/include/linux/pid.h)
 
-use crate::{bindings, sync::aref::AlwaysRefCounted, types::Opaque};
+use crate::{
+    bindings,
+    sync::aref::RefCounted,
+    types::{AlwaysRefCounted, Opaque},
+};
 use core::ptr;
 
 /// Wraps the kernel's `struct pid_namespace`. Thread safe.
@@ -41,7 +45,7 @@ pub unsafe fn from_ptr<'a>(ptr: *const bindings::pid_namespace) -> &'a Self {
 }
 
 // SAFETY: Instances of `PidNamespace` are always reference-counted.
-unsafe impl AlwaysRefCounted for PidNamespace {
+unsafe impl RefCounted for PidNamespace {
     #[inline]
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
@@ -55,6 +59,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<PidNamespace>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<PidNamespace>` from
+// a `&PidNamespace`.
+unsafe impl AlwaysRefCounted for PidNamespace {}
+
 // SAFETY:
 // - `PidNamespace::dec_ref` can be called from any thread.
 // - It is okay to send ownership of `PidNamespace` across thread boundaries.
diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
index 35a5813ffb33f..139517c21961e 100644
--- a/rust/kernel/platform.rs
+++ b/rust/kernel/platform.rs
@@ -13,6 +13,7 @@
     irq::{self, IrqRequest},
     of,
     prelude::*,
+    sync::aref::{AlwaysRefCounted, RefCounted},
     types::Opaque,
     ThisModule,
 };
@@ -490,7 +491,7 @@ pub fn optional_irq_by_name(&self, name: &CStr) -> Result<IrqRequest<'_>> {
 impl crate::dma::Device for Device<device::Core> {}
 
 // SAFETY: Instances of `Device` are always reference-counted.
-unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
+unsafe impl RefCounted for Device {
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
         unsafe { bindings::get_device(self.as_ref().as_raw()) };
@@ -502,6 +503,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
+// `&Device`.
+unsafe impl AlwaysRefCounted for Device {}
+
 impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
     fn as_ref(&self) -> &device::Device<Ctx> {
         // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index e175aefe86151..61caddfd89619 100644
--- a/rust/kernel/sync/aref.rs
+++ b/rust/kernel/sync/aref.rs
@@ -19,11 +19,9 @@
 
 use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref, ptr::NonNull};
 
-/// Types that are _always_ reference counted.
+/// Types that are internally reference counted.
 ///
 /// It allows such types to define their own custom ref increment and decrement functions.
-/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
-/// [`ARef<T>`].
 ///
 /// This is usually implemented by wrappers to existing structures on the C side of the code. For
 /// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
@@ -40,9 +38,8 @@
 /// at least until matching decrements are performed.
 ///
 /// Implementers must also ensure that all instances are reference-counted. (Otherwise they
-/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
-/// alive.)
-pub unsafe trait AlwaysRefCounted {
+/// won't be able to honour the requirement that [`RefCounted::inc_ref`] keep the object alive.)
+pub unsafe trait RefCounted {
     /// Increments the reference count on the object.
     fn inc_ref(&self);
 
@@ -55,11 +52,27 @@ pub unsafe trait AlwaysRefCounted {
     /// Callers must ensure that there was a previous matching increment to the reference count,
     /// and that the object is no longer used after its reference count is decremented (as it may
     /// result in the object being freed), unless the caller owns another increment on the refcount
-    /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
-    /// [`AlwaysRefCounted::dec_ref`] once).
+    /// (e.g., it calls [`RefCounted::inc_ref`] twice, then calls [`RefCounted::dec_ref`] once).
     unsafe fn dec_ref(obj: NonNull<Self>);
 }
 
+/// Always reference-counted type.
+///
+/// It allows deriving a counted reference [`ARef<T>`] from a `&T`.
+///
+/// This provides some convenience, but it allows "escaping" borrow checks on `&T`. As it
+/// complicates attempts to ensure that a reference to T is unique, it is optional to provide for
+/// [`RefCounted`] types. See *Safety* below.
+///
+/// # Safety
+///
+/// Implementers must ensure that no safety invariants are violated by upgrading an `&T` to an
+/// [`ARef<T>`]. In particular that implies [`AlwaysRefCounted`] and [`crate::types::Ownable`]
+/// cannot be implemented for the same type, as this would allow violating the uniqueness guarantee
+/// of [`crate::types::Owned<T>`] by dereferencing it into an `&T` and obtaining an [`ARef`] from
+/// that.
+pub unsafe trait AlwaysRefCounted: RefCounted {}
+
 /// An owned reference to an always-reference-counted object.
 ///
 /// The object's reference count is automatically decremented when an instance of [`ARef`] is
@@ -70,7 +83,7 @@ pub unsafe trait AlwaysRefCounted {
 ///
 /// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
 /// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
-pub struct ARef<T: AlwaysRefCounted> {
+pub struct ARef<T: RefCounted> {
     ptr: NonNull<T>,
     _p: PhantomData<T>,
 }
@@ -79,16 +92,16 @@ pub struct ARef<T: AlwaysRefCounted> {
 // it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
 // `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
 // mutable reference, for example, when the reference count reaches zero and `T` is dropped.
-unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
+unsafe impl<T: RefCounted + Sync + Send> Send for ARef<T> {}
 
 // SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
 // because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
 // it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
 // `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
 // example, when the reference count reaches zero and `T` is dropped.
-unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
+unsafe impl<T: RefCounted + Sync + Send> Sync for ARef<T> {}
 
-impl<T: AlwaysRefCounted> ARef<T> {
+impl<T: RefCounted> ARef<T> {
     /// Creates a new instance of [`ARef`].
     ///
     /// It takes over an increment of the reference count on the underlying object.
@@ -117,12 +130,12 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
     ///
     /// ```
     /// use core::ptr::NonNull;
-    /// use kernel::sync::aref::{ARef, AlwaysRefCounted};
+    /// use kernel::sync::aref::{ARef, RefCounted};
     ///
     /// struct Empty {}
     ///
     /// # // SAFETY: TODO.
-    /// unsafe impl AlwaysRefCounted for Empty {
+    /// unsafe impl RefCounted for Empty {
     ///     fn inc_ref(&self) {}
     ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
     /// }
@@ -140,7 +153,7 @@ pub fn into_raw(me: Self) -> NonNull<T> {
     }
 }
 
-impl<T: AlwaysRefCounted> Clone for ARef<T> {
+impl<T: RefCounted> Clone for ARef<T> {
     fn clone(&self) -> Self {
         self.inc_ref();
         // SAFETY: We just incremented the refcount above.
@@ -148,7 +161,7 @@ fn clone(&self) -> Self {
     }
 }
 
-impl<T: AlwaysRefCounted> Deref for ARef<T> {
+impl<T: RefCounted> Deref for ARef<T> {
     type Target = T;
 
     fn deref(&self) -> &Self::Target {
@@ -165,7 +178,7 @@ fn from(b: &T) -> Self {
     }
 }
 
-impl<T: AlwaysRefCounted> Drop for ARef<T> {
+impl<T: RefCounted> Drop for ARef<T> {
     fn drop(&mut self) {
         // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
         // decrement.
diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 49fad6de06740..0a6e38d984560 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -9,8 +9,8 @@
     ffi::{c_int, c_long, c_uint},
     mm::MmWithUser,
     pid_namespace::PidNamespace,
-    sync::aref::ARef,
-    types::{NotThreadSafe, Opaque},
+    sync::aref::{AlwaysRefCounted, RefCounted},
+    types::{ARef, NotThreadSafe, Opaque},
 };
 use core::{
     cmp::{Eq, PartialEq},
@@ -348,7 +348,7 @@ pub fn active_pid_ns(&self) -> Option<&PidNamespace> {
 }
 
 // SAFETY: The type invariants guarantee that `Task` is always refcounted.
-unsafe impl crate::sync::aref::AlwaysRefCounted for Task {
+unsafe impl RefCounted for Task {
     #[inline]
     fn inc_ref(&self) {
         // SAFETY: The existence of a shared reference means that the refcount is nonzero.
@@ -362,6 +362,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
     }
 }
 
+// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Task>` from a
+// `&Task`.
+unsafe impl AlwaysRefCounted for Task {}
+
 impl Kuid {
     /// Get the current euid.
     #[inline]
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 4aec7b699269a..9b96aa2ebdb7e 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -18,7 +18,8 @@
     },
     sync::aref::{
         ARef,
-        AlwaysRefCounted, //
+        AlwaysRefCounted,
+        RefCounted, //
     }, //
 };
 

-- 
2.51.2

[PATCH v14 3/9] rust: Add missing SAFETY documentation for `ARef` example

[Andreas Hindborg]

From: Oliver Mangold <oliver.mangold@pm.me>

SAFETY comment in rustdoc example was just 'TODO'. Fixed.

Original patch by Oliver Mangold <oliver.mangold@pm.me> [1].

Link: https://lore.kernel.org/r/20251117-unique-ref-v13-3-b5b243df1250@pm.me [1]
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/sync/aref.rs | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index 61caddfd89619..efe16a7fdfa5d 100644
--- a/rust/kernel/sync/aref.rs
+++ b/rust/kernel/sync/aref.rs
@@ -129,12 +129,14 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
     /// # Examples
     ///
     /// ```
-    /// use core::ptr::NonNull;
-    /// use kernel::sync::aref::{ARef, RefCounted};
+    /// # use core::ptr::NonNull;
+    /// # use kernel::sync::aref::{ARef, RefCounted};
     ///
     /// struct Empty {}
     ///
-    /// # // SAFETY: TODO.
+    /// // SAFETY: The `RefCounted` implementation for `Empty` does not count references and never
+    /// // frees the underlying object. Thus we can act as owning an increment on the refcount for
+    /// // the object that we pass to the newly created `ARef`.
     /// unsafe impl RefCounted for Empty {
     ///     fn inc_ref(&self) {}
     ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
@@ -142,7 +144,7 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
     ///
     /// let mut data = Empty {};
     /// let ptr = NonNull::<Empty>::new(&mut data).unwrap();
-    /// # // SAFETY: TODO.
+    /// // SAFETY: We keep `data` around longer than the `ARef`.
     /// let data_ref: ARef<Empty> = unsafe { ARef::from_raw(ptr) };
     /// let raw_ptr: NonNull<Empty> = ARef::into_raw(data_ref);
     ///

-- 
2.51.2

[PATCH v14 4/9] rust: aref: update formatting of use statements

[Andreas Hindborg]

Update formatting if use statements in preparation for next commit.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/sync/aref.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index efe16a7fdfa5d..3c63c9a5fb9be 100644
--- a/rust/kernel/sync/aref.rs
+++ b/rust/kernel/sync/aref.rs
@@ -17,7 +17,12 @@
 //! [`Arc`]: crate::sync::Arc
 //! [`Arc<T>`]: crate::sync::Arc
 
-use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref, ptr::NonNull};
+use core::{
+    marker::PhantomData,
+    mem::ManuallyDrop,
+    ops::Deref,
+    ptr::NonNull, //
+};
 
 /// Types that are internally reference counted.
 ///

-- 
2.51.2

[PATCH v14 5/9] rust: Add `OwnableRefCounted`

[Andreas Hindborg]

From: Oliver Mangold <oliver.mangold@pm.me>

Types implementing one of these traits can safely convert between an
`ARef<T>` and an `Owned<T>`.

This is useful for types which generally are accessed through an `ARef`
but have methods which can only safely be called when the reference is
unique, like e.g. `block::mq::Request::end_ok()`.

Original patch by Oliver Mangold <oliver.mangold@pm.me> [1].

Link: https://lore.kernel.org/r/20251117-unique-ref-v13-4-b5b243df1250@pm.me [1]
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/owned.rs     | 143 ++++++++++++++++++++++++++++++++++++++++++++---
 rust/kernel/sync/aref.rs |  15 ++++-
 rust/kernel/types.rs     |   1 +
 3 files changed, 150 insertions(+), 9 deletions(-)

diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
index b02edda11fcf6..85251c57f86c6 100644
--- a/rust/kernel/owned.rs
+++ b/rust/kernel/owned.rs
@@ -14,18 +14,24 @@
     pin::Pin,
     ptr::NonNull, //
 };
+use kernel::{
+    sync::aref::ARef,
+    types::RefCounted, //
+};
 
 /// Types that specify their own way of performing allocation and destruction. Typically, this trait
 /// is implemented on types from the C side.
 ///
-/// Implementing this trait allows types to be referenced via the [`Owned<Self>`] pointer type. This
-/// is useful when it is desirable to tie the lifetime of the reference to an owned object, rather
-/// than pass around a bare reference. [`Ownable`] types can define custom drop logic that is
-/// executed when the owned reference [`Owned<Self>`] pointing to the object is dropped.
+/// Implementing this trait allows types to be referenced via the [`Owned<Self>`] pointer type.
+///  - This is useful when it is desirable to tie the lifetime of an object reference to an owned
+///    object, rather than pass around a bare reference.
+///  - [`Ownable`] types can define custom drop logic that is executed when the owned reference
+///    of type [`Owned<_>`] pointing to the object is dropped.
 ///
 /// Note: The underlying object is not required to provide internal reference counting, because it
 /// represents a unique, owned reference. If reference counting (on the Rust side) is required,
-/// [`RefCounted`](crate::types::RefCounted) should be implemented.
+/// [`RefCounted`] should be implemented. [`OwnableRefCounted`] should be implemented if conversion
+/// between unique and shared (reference counted) ownership is needed.
 ///
 /// # Safety
 ///
@@ -63,8 +69,7 @@
 ///             Foo {},
 ///             flags::GFP_KERNEL,
 ///         )?;
-///         let result = NonNull::new(KBox::into_raw(result))
-///             .expect("Raw pointer to newly allocation KBox is null, this should never happen.");
+///         let result = NonNull::new(KBox::into_raw(result)).ok_or(ENOMEM)?;
 ///         // Count new allocation
 ///         *FOO_ALLOC_COUNT.lock() += 1;
 ///         // SAFETY: We just allocated the `Self`, thus it is valid and there cannot be any other
@@ -88,11 +93,12 @@
 /// }
 ///
 /// {
-///    let foo = Foo::new().expect("Failed to allocate a Foo. This shouldn't happen");
+///    let foo = Foo::new()?;
 ///    assert!(*FOO_ALLOC_COUNT.lock() == 1);
 /// }
 /// // `foo` is out of scope now, so we expect no live allocations.
 /// assert!(*FOO_ALLOC_COUNT.lock() == 0);
+/// # Ok::<(), Error>(())
 /// ```
 pub unsafe trait Ownable {
     /// Releases the object.
@@ -194,3 +200,124 @@ fn drop(&mut self) {
         unsafe { T::release(self.ptr) };
     }
 }
+
+/// A trait for objects that can be wrapped in either one of the reference types [`Owned`] and
+/// [`ARef`].
+///
+/// # Examples
+///
+/// A minimal example implementation of [`OwnableRefCounted`], [`Ownable`] and its usage with
+/// [`ARef`] and [`Owned`] looks like this:
+///
+/// ```
+/// # #![expect(clippy::disallowed_names)]
+/// # use core::cell::Cell;
+/// # use core::ptr::NonNull;
+/// # use kernel::alloc::{flags, kbox::KBox, AllocError};
+/// # use kernel::sync::aref::{ARef, RefCounted};
+/// # use kernel::types::{Owned, Ownable, OwnableRefCounted};
+///
+/// // An internally refcounted struct for demonstration purposes.
+/// //
+/// // # Invariants
+/// //
+/// // - `refcount` is always non-zero for a valid object.
+/// // - `refcount` is >1 if there is more than one Rust reference to it.
+/// //
+/// struct Foo {
+///     refcount: Cell<usize>,
+/// }
+///
+/// impl Foo {
+///     fn new() -> Result<Owned<Self>> {
+///         // We are just using a `KBox` here to handle the actual allocation, as our `Foo` is
+///         // not actually a C-allocated object.
+///         let result = KBox::new(
+///             Foo {
+///                 refcount: Cell::new(1),
+///             },
+///             flags::GFP_KERNEL,
+///         )?;
+///         let result = NonNull::new(KBox::into_raw(result)).ok_or(ENOMEM)?;
+///         // SAFETY: We just allocated the `Self`, thus it is valid and there cannot be any other
+///         // Rust references. Calling `into_raw()` makes us responsible for ownership and
+///         // we won't use the raw pointer anymore, thus we can transfer ownership to the `Owned`.
+///         Ok(unsafe { Owned::from_raw(result) })
+///     }
+/// }
+///
+/// // SAFETY: We increment and decrement each time the respective function is called and only free
+/// // the `Foo` when the refcount reaches zero.
+/// unsafe impl RefCounted for Foo {
+///     fn inc_ref(&self) {
+///         self.refcount.replace(self.refcount.get() + 1);
+///     }
+///
+///     unsafe fn dec_ref(this: NonNull<Self>) {
+///         // SAFETY: By requirement on calling this function, the refcount is non-zero,
+///         // implying the underlying object is valid.
+///         let refcount = unsafe { &this.as_ref().refcount };
+///         let new_refcount = refcount.get() - 1;
+///         if new_refcount == 0 {
+///             // The `Foo` will be dropped when `KBox` goes out of scope.
+///             // SAFETY: The [`KBox<Foo>`] is still alive as the old refcount is 1. We can pass
+///             // ownership to the [`KBox`] as by requirement on calling this function,
+///             // the `Self` will no longer be used by the caller.
+///             unsafe { KBox::from_raw(this.as_ptr()) };
+///         } else {
+///             refcount.replace(new_refcount);
+///         }
+///     }
+/// }
+///
+/// impl OwnableRefCounted for Foo {
+///     fn try_from_shared(this: ARef<Self>) -> Result<Owned<Self>, ARef<Self>> {
+///         if this.refcount.get() == 1 {
+///             // SAFETY: The `Foo` is still alive and has no other Rust references as the refcount
+///             // is 1.
+///             Ok(unsafe { Owned::from_raw(ARef::into_raw(this)) })
+///         } else {
+///             Err(this)
+///         }
+///     }
+/// }
+///
+/// // SAFETY: This implementation of `release()` is safe for any valid `Self`.
+/// unsafe impl Ownable for Foo {
+///     unsafe fn release(this: NonNull<Self>) {
+///         // SAFETY: Using `dec_ref()` from [`RefCounted`] to release is okay, as the refcount is
+///         // always 1 for an [`Owned<Foo>`].
+///         unsafe{ Foo::dec_ref(this) };
+///     }
+/// }
+///
+/// let foo = Foo::new()?;
+/// let mut foo = ARef::from(foo);
+/// {
+///     let bar = foo.clone();
+///     assert!(Owned::try_from(bar).is_err());
+/// }
+/// assert!(Owned::try_from(foo).is_ok());
+/// # Ok::<(), Error>(())
+/// ```
+pub trait OwnableRefCounted: RefCounted + Ownable + Sized {
+    /// Checks if the [`ARef`] is unique and converts it to an [`Owned`] if that is the case.
+    /// Otherwise it returns again an [`ARef`] to the same underlying object.
+    fn try_from_shared(this: ARef<Self>) -> Result<Owned<Self>, ARef<Self>>;
+
+    /// Converts the [`Owned`] into an [`ARef`].
+    fn into_shared(this: Owned<Self>) -> ARef<Self> {
+        // SAFETY: Safe by the requirements on implementing the trait.
+        unsafe { ARef::from_raw(Owned::into_raw(this)) }
+    }
+}
+
+impl<T: OwnableRefCounted> TryFrom<ARef<T>> for Owned<T> {
+    type Error = ARef<T>;
+    /// Tries to convert the [`ARef`] to an [`Owned`] by calling
+    /// [`try_from_shared()`](OwnableRefCounted::try_from_shared). In case the [`ARef`] is not
+    /// unique, it returns again an [`ARef`] to the same underlying object.
+    fn try_from(b: ARef<T>) -> Result<Owned<T>, Self::Error> {
+        T::try_from_shared(b)
+    }
+}
diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
index 3c63c9a5fb9be..77f6c8dc411eb 100644
--- a/rust/kernel/sync/aref.rs
+++ b/rust/kernel/sync/aref.rs
@@ -23,6 +23,10 @@
     ops::Deref,
     ptr::NonNull, //
 };
+use kernel::types::{
+    OwnableRefCounted,
+    Owned, //
+};
 
 /// Types that are internally reference counted.
 ///
@@ -35,7 +39,10 @@
 /// Note: Implementing this trait allows types to be wrapped in an [`ARef<Self>`]. It requires an
 /// internal reference count and provides only shared references. If unique references are required
 /// [`Ownable`](crate::types::Ownable) should be implemented which allows types to be wrapped in an
-/// [`Owned<Self>`](crate::types::Owned).
+/// [`Owned<Self>`](crate::types::Owned). Implementing the trait
+/// [`OwnableRefCounted`] allows to convert between unique and
+/// shared references (i.e. [`Owned<Self>`](crate::types::Owned) and
+/// [`ARef<Self>`](crate::types::Owned)).
 ///
 /// # Safety
 ///
@@ -185,6 +192,12 @@ fn from(b: &T) -> Self {
     }
 }
 
+impl<T: OwnableRefCounted> From<Owned<T>> for ARef<T> {
+    fn from(b: Owned<T>) -> Self {
+        T::into_shared(b)
+    }
+}
+
 impl<T: RefCounted> Drop for ARef<T> {
     fn drop(&mut self) {
         // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 9b96aa2ebdb7e..f43c091eeb8b7 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -14,6 +14,7 @@
 pub use crate::{
     owned::{
         Ownable,
+        OwnableRefCounted,
         Owned, //
     },
     sync::aref::{

-- 
2.51.2

[PATCH v14 6/9] rust: page: update formatting of `use` statements

[Andreas Hindborg]

Update formatting in preparation for next patch

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/page.rs | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index 432fc0297d4a8..bf3bed7e2d3fe 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -3,17 +3,23 @@
 //! Kernel page allocation and management.
 
 use crate::{
-    alloc::{AllocError, Flags},
+    alloc::{
+        AllocError,
+        Flags, //
+    },
     bindings,
     error::code::*,
     error::Result,
-    uaccess::UserSliceReader,
+    uaccess::UserSliceReader, //
 };
 use core::{
     marker::PhantomData,
     mem::ManuallyDrop,
     ops::Deref,
-    ptr::{self, NonNull},
+    ptr::{
+        self,
+        NonNull, //
+    }, //
 };
 
 /// A bitwise shift for the page size.

-- 
2.51.2

[PATCH v14 7/9] rust: page: convert to `Ownable`

[Andreas Hindborg]

From: Asahi Lina <lina+kernel@asahilina.net>

This allows Page references to be returned as borrowed references,
without necessarily owning the struct page.

Original patch by Asahi Lina <lina@asahilina.net> [1].

Link: https://lore.kernel.org/rust-for-linux/20250202-rust-page-v1-2-e3170d7fe55e@asahilina.net/ [1]
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/page.rs | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index bf3bed7e2d3fe..4591b7b01c3d2 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -10,6 +10,11 @@
     bindings,
     error::code::*,
     error::Result,
+    types::{
+        Opaque,
+        Ownable,
+        Owned, //
+    },
     uaccess::UserSliceReader, //
 };
 use core::{
@@ -83,7 +88,7 @@ pub const fn page_align(addr: usize) -> usize {
 ///
 /// [`VBox`]: kernel::alloc::VBox
 /// [`Vmalloc`]: kernel::alloc::allocator::Vmalloc
-pub struct BorrowedPage<'a>(ManuallyDrop<Page>, PhantomData<&'a Page>);
+pub struct BorrowedPage<'a>(ManuallyDrop<Owned<Page>>, PhantomData<&'a Owned<Page>>);
 
 impl<'a> BorrowedPage<'a> {
     /// Constructs a [`BorrowedPage`] from a raw pointer to a `struct page`.
@@ -93,7 +98,9 @@ impl<'a> BorrowedPage<'a> {
     /// - `ptr` must point to a valid `bindings::page`.
     /// - `ptr` must remain valid for the entire lifetime `'a`.
     pub unsafe fn from_raw(ptr: NonNull<bindings::page>) -> Self {
-        let page = Page { page: ptr };
+        let page: Owned<Page> =
+            // SAFETY: By function safety requirements `ptr` is non null and valid for 'a.
+            unsafe { Owned::from_raw(NonNull::new_unchecked(ptr.as_ptr().cast())) };
 
         // INVARIANT: The safety requirements guarantee that `ptr` is valid for the entire lifetime
         // `'a`.
@@ -126,8 +133,9 @@ pub trait AsPageIter {
 /// # Invariants
 ///
 /// The pointer is valid, and has ownership over the page.
+#[repr(transparent)]
 pub struct Page {
-    page: NonNull<bindings::page>,
+    page: Opaque<bindings::page>,
 }
 
 // SAFETY: Pages have no logic that relies on them staying on a given thread, so moving them across
@@ -161,19 +169,20 @@ impl Page {
     /// # Ok::<(), kernel::alloc::AllocError>(())
     /// ```
     #[inline]
-    pub fn alloc_page(flags: Flags) -> Result<Self, AllocError> {
+    pub fn alloc_page(flags: Flags) -> Result<Owned<Self>, AllocError> {
         // SAFETY: Depending on the value of `gfp_flags`, this call may sleep. Other than that, it
         // is always safe to call this method.
         let page = unsafe { bindings::alloc_pages(flags.as_raw(), 0) };
         let page = NonNull::new(page).ok_or(AllocError)?;
-        // INVARIANT: We just successfully allocated a page, so we now have ownership of the newly
-        // allocated page. We transfer that ownership to the new `Page` object.
-        Ok(Self { page })
+        // SAFETY: We just successfully allocated a page, so we now have ownership of the newly
+        // allocated page. We transfer that ownership to the new `Owned<Page>` object.
+        // Since `Page` is transparent, we can cast the pointer directly.
+        Ok(unsafe { Owned::from_raw(page.cast()) })
     }
 
     /// Returns a raw pointer to the page.
     pub fn as_ptr(&self) -> *mut bindings::page {
-        self.page.as_ptr()
+        Opaque::cast_into(&self.page)
     }
 
     /// Get the node id containing this page.
@@ -348,10 +357,13 @@ pub unsafe fn copy_from_user_slice_raw(
     }
 }
 
-impl Drop for Page {
+// SAFETY: `Owned<Page>` objects returned by Page::alloc_page() follow the requirements of
+// the Ownable abstraction.
+unsafe impl Ownable for Page {
     #[inline]
-    fn drop(&mut self) {
+    unsafe fn release(this: NonNull<Self>) {
         // SAFETY: By the type invariants, we have ownership of the page and can free it.
-        unsafe { bindings::__free_pages(self.page.as_ptr(), 0) };
+        // Since Page is transparent, we can cast the raw pointer directly.
+        unsafe { bindings::__free_pages(this.cast().as_ptr(), 0) };
     }
 }

-- 
2.51.2

[PATCH v14 8/9] rust: implement `ForeignOwnable` for `Owned`

[Andreas Hindborg]

Implement `ForeignOwnable` for `Owned<T>`. This allows use of `Owned<T>` in
places such as the `XArray`.

Note that `T` does not need to implement `ForeignOwnable` for `Owned<T>` to
implement `ForeignOwnable`.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/owned.rs | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
index 85251c57f86c6..0b22de4aaf584 100644
--- a/rust/kernel/owned.rs
+++ b/rust/kernel/owned.rs
@@ -16,7 +16,10 @@
 };
 use kernel::{
     sync::aref::ARef,
-    types::RefCounted, //
+    types::{
+        ForeignOwnable, //
+        RefCounted,
+    }, //
 };
 
 /// Types that specify their own way of performing allocation and destruction. Typically, this trait
@@ -120,6 +123,7 @@ pub unsafe trait Ownable {
 ///
 /// - The [`Owned<T>`] has exclusive access to the instance of `T`.
 /// - The instance of `T` will stay alive at least as long as the [`Owned<T>`] is alive.
+#[repr(transparent)]
 pub struct Owned<T: Ownable> {
     ptr: NonNull<T>,
 }
@@ -201,6 +205,45 @@ fn drop(&mut self) {
     }
 }
 
+// SAFETY: We derive the pointer to `T` from a valid `T`, so the returned
+// pointer satisfy alignment requirements of `T`.
+unsafe impl<T: Ownable + 'static> ForeignOwnable for Owned<T> {
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<Owned<T>>();
+
+    type Borrowed<'a> = &'a T;
+    type BorrowedMut<'a> = Pin<&'a mut T>;
+
+    fn into_foreign(self) -> *mut kernel::ffi::c_void {
+        let ptr = self.ptr.as_ptr().cast();
+        core::mem::forget(self);
+        ptr
+    }
+
+    unsafe fn from_foreign(ptr: *mut kernel::ffi::c_void) -> Self {
+        Self {
+            // SAFETY: By function safety contract, `ptr` came from
+            // `into_foreign` and cannot be null.
+            ptr: unsafe { NonNull::new_unchecked(ptr.cast()) },
+        }
+    }
+
+    unsafe fn borrow<'a>(ptr: *mut kernel::ffi::c_void) -> Self::Borrowed<'a> {
+        // SAFETY: By function safety requirements, `ptr` is valid for use as a
+        // reference for `'a`.
+        unsafe { &*ptr.cast() }
+    }
+
+    unsafe fn borrow_mut<'a>(ptr: *mut kernel::ffi::c_void) -> Self::BorrowedMut<'a> {
+        // SAFETY: By function safety requirements, `ptr` is valid for use as a
+        // unique reference for `'a`.
+        let inner = unsafe { &mut *ptr.cast() };
+
+        // SAFETY: We never move out of inner, and we do not hand out mutable
+        // references when `T: !Unpin`.
+        unsafe { Pin::new_unchecked(inner) }
+    }
+}
+
 /// A trait for objects that can be wrapped in either one of the reference types [`Owned`] and
 /// [`ARef`].
 ///

-- 
2.51.2

[PATCH v14 9/9] rust: page: add `from_raw()`

[Andreas Hindborg]

Add a method to `Page` that allows construction of an instance from `struct
page` pointer.

Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>
---
 rust/kernel/page.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
index 4591b7b01c3d2..803f3e3d76b22 100644
--- a/rust/kernel/page.rs
+++ b/rust/kernel/page.rs
@@ -191,6 +191,17 @@ pub fn nid(&self) -> i32 {
         unsafe { bindings::page_to_nid(self.as_ptr()) }
     }
 
+    /// Create a `&Page` from a raw `struct page` pointer
+    ///
+    /// # Safety
+    ///
+    /// `ptr` must be valid for use as a reference for the duration of `'a`.
+    pub unsafe fn from_raw<'a>(ptr: *const bindings::page) -> &'a Self {
+        // SAFETY: By function safety requirements, ptr is not null and is
+        // valid for use as a reference.
+        unsafe { &*Opaque::cast_from(ptr).cast::<Self>() }
+    }
+
     /// Runs a piece of code with this page mapped to an address.
     ///
     /// The page is unmapped when this call returns.

-- 
2.51.2

Re: [PATCH v14 2/9] rust: rename `AlwaysRefCounted` to `RefCounted`.

[Igor Korotin]

Hello Andreas

On 2/4/2026 11:56 AM, Andreas Hindborg wrote:
> From: Oliver Mangold <oliver.mangold@pm.me>
> 
> There are types where it may both be reference counted in some cases and
> owned in others. In such cases, obtaining `ARef<T>` from `&T` would be
> unsound as it allows creation of `ARef<T>` copy from `&Owned<T>`.
> 
> Therefore, we split `AlwaysRefCounted` into `RefCounted` (which `ARef<T>`
> would require) and a marker trait to indicate that the type is always
> reference counted (and not `Ownable`) so the `&T` -> `ARef<T>` conversion
> is possible.
> 
> - Rename `AlwaysRefCounted` to `RefCounted`.
> - Add a new unsafe trait `AlwaysRefCounted`.
> - Implement the new trait `AlwaysRefCounted` for the newly renamed
>    `RefCounted` implementations. This leaves functionality of existing
>    implementers of `AlwaysRefCounted` intact.
> 
> Original patch by Oliver Mangold <oliver.mangold@pm.me> [1].
> 
> Link: https://lore.kernel.org/r/20251117-unique-ref-v13-2-b5b243df1250@pm.me [1]
> Suggested-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>   rust/kernel/auxiliary.rs        |  7 +++++-
>   rust/kernel/block/mq/request.rs | 15 +++++++------
>   rust/kernel/cred.rs             | 13 ++++++++++--
>   rust/kernel/device.rs           | 10 ++++++---
>   rust/kernel/device/property.rs  |  7 +++++-
>   rust/kernel/drm/device.rs       | 10 ++++++---
>   rust/kernel/drm/gem/mod.rs      |  8 ++++---
>   rust/kernel/fs/file.rs          | 16 ++++++++++----
>   rust/kernel/i2c.rs              | 16 +++++++++-----
>   rust/kernel/mm.rs               | 15 +++++++++----
>   rust/kernel/mm/mmput_async.rs   |  9 ++++++--
>   rust/kernel/opp.rs              | 10 ++++++---
>   rust/kernel/owned.rs            |  2 +-
>   rust/kernel/pci.rs              | 10 ++++++++-
>   rust/kernel/pid_namespace.rs    | 12 +++++++++--
>   rust/kernel/platform.rs         |  7 +++++-
>   rust/kernel/sync/aref.rs        | 47 ++++++++++++++++++++++++++---------------
>   rust/kernel/task.rs             | 10 ++++++---
>   rust/kernel/types.rs            |  3 ++-
>   19 files changed, 164 insertions(+), 63 deletions(-)
> 
> diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
> index be76f11aecb7e..c410dcfc7b6f7 100644
> --- a/rust/kernel/auxiliary.rs
> +++ b/rust/kernel/auxiliary.rs
> @@ -11,6 +11,7 @@
>       driver,
>       error::{from_result, to_result, Result},
>       prelude::*,
> +    sync::aref::{AlwaysRefCounted, RefCounted},
>       types::Opaque,
>       ThisModule,
>   };
> @@ -283,7 +284,7 @@ unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for Device<Ctx>
>   kernel::impl_device_context_into_aref!(Device);
>   
>   // SAFETY: Instances of `Device` are always reference-counted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
> +unsafe impl RefCounted for Device {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>           unsafe { bindings::get_device(self.as_ref().as_raw()) };
> @@ -302,6 +303,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl AlwaysRefCounted for Device {}
> +
>   impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
>       fn as_ref(&self) -> &device::Device<Ctx> {
>           // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
> index ce3e30c81cb5e..cf013b9e2cacf 100644
> --- a/rust/kernel/block/mq/request.rs
> +++ b/rust/kernel/block/mq/request.rs
> @@ -9,7 +9,7 @@
>       block::mq::Operations,
>       error::Result,
>       sync::{
> -        aref::{ARef, AlwaysRefCounted},
> +        aref::{ARef, AlwaysRefCounted, RefCounted},
>           atomic::Relaxed,
>           Refcount,
>       },
> @@ -229,11 +229,10 @@ unsafe impl<T: Operations> Send for Request<T> {}
>   // mutate `self` are internally synchronized`
>   unsafe impl<T: Operations> Sync for Request<T> {}
>   
> -// SAFETY: All instances of `Request<T>` are reference counted. This
> -// implementation of `AlwaysRefCounted` ensure that increments to the ref count
> -// keeps the object alive in memory at least until a matching reference count
> -// decrement is executed.
> -unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {
> +// SAFETY: All instances of `Request<T>` are reference counted. This implementation of `RefCounted`
> +// ensure that increments to the ref count keeps the object alive in memory at least until a
> +// matching reference count decrement is executed.
> +unsafe impl<T: Operations> RefCounted for Request<T> {
>       fn inc_ref(&self) {
>           self.wrapper_ref().refcount().inc();
>       }
> @@ -255,3 +254,7 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) {
>           }
>       }
>   }
> +
> +// SAFETY: We currently do not implement `Ownable`, thus it is okay to obtain an `ARef<Request>`
> +// from a `&Request` (but this will change in the future).
> +unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {}
> diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
> index ffa156b9df377..20ef0144094be 100644
> --- a/rust/kernel/cred.rs
> +++ b/rust/kernel/cred.rs
> @@ -8,7 +8,12 @@
>   //!
>   //! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
>   
> -use crate::{bindings, sync::aref::AlwaysRefCounted, task::Kuid, types::Opaque};
> +use crate::{
> +    bindings,
> +    sync::aref::RefCounted,
> +    task::Kuid,
> +    types::{AlwaysRefCounted, Opaque},
> +};
>   
>   /// Wraps the kernel's `struct cred`.
>   ///
> @@ -76,7 +81,7 @@ pub fn euid(&self) -> Kuid {
>   }
>   
>   // SAFETY: The type invariants guarantee that `Credential` is always ref-counted.
> -unsafe impl AlwaysRefCounted for Credential {
> +unsafe impl RefCounted for Credential {
>       #[inline]
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -90,3 +95,7 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) {
>           unsafe { bindings::put_cred(obj.cast().as_ptr()) };
>       }
>   }
> +
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Credential>` from a
> +// `&Credential`.
> +unsafe impl AlwaysRefCounted for Credential {}
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index 031720bf5d8ca..e09dad5f9afea 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -7,8 +7,8 @@
>   use crate::{
>       bindings, fmt,
>       prelude::*,
> -    sync::aref::ARef,
> -    types::{ForeignOwnable, Opaque},
> +    sync::aref::{ARef, RefCounted},
> +    types::{AlwaysRefCounted, ForeignOwnable, Opaque},
>   };
>   use core::{any::TypeId, marker::PhantomData, ptr};
>   
> @@ -492,7 +492,7 @@ pub fn fwnode(&self) -> Option<&property::FwNode> {
>   kernel::impl_device_context_into_aref!(Device);
>   
>   // SAFETY: Instances of `Device` are always reference-counted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
> +unsafe impl RefCounted for Device {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>           unsafe { bindings::get_device(self.as_raw()) };
> @@ -504,6 +504,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl AlwaysRefCounted for Device {}
> +
>   // SAFETY: As by the type invariant `Device` can be sent to any thread.
>   unsafe impl Send for Device {}
>   
> diff --git a/rust/kernel/device/property.rs b/rust/kernel/device/property.rs
> index 3a332a8c53a9e..a8bb824ad0ec1 100644
> --- a/rust/kernel/device/property.rs
> +++ b/rust/kernel/device/property.rs
> @@ -14,6 +14,7 @@
>       fmt,
>       prelude::*,
>       str::{CStr, CString},
> +    sync::aref::{AlwaysRefCounted, RefCounted},
>       types::{ARef, Opaque},
>   };
>   
> @@ -359,7 +360,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
>   }
>   
>   // SAFETY: Instances of `FwNode` are always reference-counted.
> -unsafe impl crate::types::AlwaysRefCounted for FwNode {
> +unsafe impl RefCounted for FwNode {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the
>           // refcount is non-zero.
> @@ -373,6 +374,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<FwNode>` from a
> +// `&FwNode`.
> +unsafe impl AlwaysRefCounted for FwNode {}
> +
>   enum Node<'a> {
>       Borrowed(&'a FwNode),
>       Owned(ARef<FwNode>),
> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
> index 3ce8f62a00569..38ce7f389ed00 100644
> --- a/rust/kernel/drm/device.rs
> +++ b/rust/kernel/drm/device.rs
> @@ -11,8 +11,8 @@
>       error::from_err_ptr,
>       error::Result,
>       prelude::*,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::Opaque,
> +    sync::aref::{AlwaysRefCounted, RefCounted},
> +    types::{ARef, Opaque},
>   };
>   use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
>   
> @@ -198,7 +198,7 @@ fn deref(&self) -> &Self::Target {
>   
>   // SAFETY: DRM device objects are always reference counted and the get/put functions
>   // satisfy the requirements.
> -unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {
> +unsafe impl<T: drm::Driver> RefCounted for Device<T> {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>           unsafe { bindings::drm_dev_get(self.as_raw()) };
> @@ -213,6 +213,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {}
> +
>   impl<T: drm::Driver> AsRef<device::Device> for Device<T> {
>       fn as_ref(&self) -> &device::Device {
>           // SAFETY: `bindings::drm_device::dev` is valid as long as the DRM device itself is valid,
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index a7f682e95c018..ad6840a440165 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -10,8 +10,7 @@
>       drm::driver::{AllocImpl, AllocOps},
>       error::{to_result, Result},
>       prelude::*,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::Opaque,
> +    types::{ARef, AlwaysRefCounted, Opaque},
>   };
>   use core::{ops::Deref, ptr::NonNull};
>   
> @@ -253,7 +252,7 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
>   }
>   
>   // SAFETY: Instances of `Object<T>` are always reference-counted.
> -unsafe impl<T: DriverObject> crate::types::AlwaysRefCounted for Object<T> {
> +unsafe impl<T: DriverObject> crate::types::RefCounted for Object<T> {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>           unsafe { bindings::drm_gem_object_get(self.as_raw()) };
> @@ -267,6 +266,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>           unsafe { bindings::drm_gem_object_put(obj.as_raw()) }
>       }
>   }
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Object`.
> +unsafe impl<T: DriverObject> crate::types::AlwaysRefCounted for Object<T> {}
>   
>   impl<T: DriverObject> super::private::Sealed for Object<T> {}
>   
> diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
> index 23ee689bd2400..06e457d62a939 100644
> --- a/rust/kernel/fs/file.rs
> +++ b/rust/kernel/fs/file.rs
> @@ -12,8 +12,8 @@
>       cred::Credential,
>       error::{code::*, to_result, Error, Result},
>       fmt,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::{NotThreadSafe, Opaque},
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
>   };
>   use core::ptr;
>   
> @@ -197,7 +197,7 @@ unsafe impl Sync for File {}
>   
>   // SAFETY: The type invariants guarantee that `File` is always ref-counted. This implementation
>   // makes `ARef<File>` own a normal refcount.
> -unsafe impl AlwaysRefCounted for File {
> +unsafe impl RefCounted for File {
>       #[inline]
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -212,6 +212,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<File>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<File>` from a
> +// `&File`.
> +unsafe impl AlwaysRefCounted for File {}
> +
>   /// Wraps the kernel's `struct file`. Not thread safe.
>   ///
>   /// This type represents a file that is not known to be safe to transfer across thread boundaries.
> @@ -233,7 +237,7 @@ pub struct LocalFile {
>   
>   // SAFETY: The type invariants guarantee that `LocalFile` is always ref-counted. This implementation
>   // makes `ARef<LocalFile>` own a normal refcount.
> -unsafe impl AlwaysRefCounted for LocalFile {
> +unsafe impl RefCounted for LocalFile {
>       #[inline]
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -249,6 +253,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<LocalFile>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<LocalFile>` from a
> +// `&LocalFile`.
> +unsafe impl AlwaysRefCounted for LocalFile {}
> +
>   impl LocalFile {
>       /// Constructs a new `struct file` wrapper from a file descriptor.
>       ///
> diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
> index 39b0a9a207fda..b5e3c236a5c16 100644
> --- a/rust/kernel/i2c.rs
> +++ b/rust/kernel/i2c.rs
> @@ -17,8 +17,10 @@
>       of,
>       prelude::*,
>       types::{
> +        ARef,
>           AlwaysRefCounted,
> -        Opaque, //
> +        Opaque,
> +        RefCounted, //
>       }, //
>   };
>   
> @@ -31,8 +33,6 @@
>       }, //
>   };
>   
> -use kernel::types::ARef;
> -
>   /// An I2C device id table.
>   #[repr(transparent)]
>   #[derive(Clone, Copy)]
> @@ -416,7 +416,7 @@ pub fn get(index: i32) -> Result<ARef<Self>> {
>   kernel::impl_device_context_into_aref!(I2cAdapter);
>   
>   // SAFETY: Instances of `I2cAdapter` are always reference-counted.
> -unsafe impl crate::types::AlwaysRefCounted for I2cAdapter {
> +unsafe impl crate::types::RefCounted for I2cAdapter {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>           unsafe { bindings::i2c_get_adapter(self.index()) };
> @@ -427,6 +427,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>           unsafe { bindings::i2c_put_adapter(obj.as_ref().as_raw()) }
>       }
>   }
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from an
> +// `&I2cAdapter`.
> +unsafe impl AlwaysRefCounted for I2cAdapter {}
>   
>   /// The i2c board info representation
>   ///
> @@ -492,7 +495,7 @@ unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for I2cClient<C
>   kernel::impl_device_context_into_aref!(I2cClient);
>   
>   // SAFETY: Instances of `I2cClient` are always reference-counted.
> -unsafe impl AlwaysRefCounted for I2cClient {
> +unsafe impl RefCounted for I2cClient {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>           unsafe { bindings::get_device(self.as_ref().as_raw()) };
> @@ -503,6 +506,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>           unsafe { bindings::put_device(&raw mut (*obj.as_ref().as_raw()).dev) }
>       }
>   }
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from an
> +// `&I2cClient`.
> +unsafe impl AlwaysRefCounted for I2cClient {}
>   
>   impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for I2cClient<Ctx> {
>       fn as_ref(&self) -> &device::Device<Ctx> {
> diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
> index 4764d7b68f2a7..dd9e3969e7206 100644
> --- a/rust/kernel/mm.rs
> +++ b/rust/kernel/mm.rs
> @@ -13,8 +13,8 @@
>   
>   use crate::{
>       bindings,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::{NotThreadSafe, Opaque},
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
>   };
>   use core::{ops::Deref, ptr::NonNull};
>   
> @@ -55,7 +55,7 @@ unsafe impl Send for Mm {}
>   unsafe impl Sync for Mm {}
>   
>   // SAFETY: By the type invariants, this type is always refcounted.
> -unsafe impl AlwaysRefCounted for Mm {
> +unsafe impl RefCounted for Mm {
>       #[inline]
>       fn inc_ref(&self) {
>           // SAFETY: The pointer is valid since self is a reference.
> @@ -69,6 +69,9 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Mm>` from a `&Mm`.
> +unsafe impl AlwaysRefCounted for Mm {}
> +
>   /// A wrapper for the kernel's `struct mm_struct`.
>   ///
>   /// This type is like [`Mm`], but with non-zero `mm_users`. It can only be used when `mm_users` can
> @@ -91,7 +94,7 @@ unsafe impl Send for MmWithUser {}
>   unsafe impl Sync for MmWithUser {}
>   
>   // SAFETY: By the type invariants, this type is always refcounted.
> -unsafe impl AlwaysRefCounted for MmWithUser {
> +unsafe impl RefCounted for MmWithUser {
>       #[inline]
>       fn inc_ref(&self) {
>           // SAFETY: The pointer is valid since self is a reference.
> @@ -105,6 +108,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<MmWithUser>` from a
> +// `&MmWithUser`.
> +unsafe impl AlwaysRefCounted for MmWithUser {}
> +
>   // Make all `Mm` methods available on `MmWithUser`.
>   impl Deref for MmWithUser {
>       type Target = Mm;
> diff --git a/rust/kernel/mm/mmput_async.rs b/rust/kernel/mm/mmput_async.rs
> index b8d2f051225c7..aba4ce675c860 100644
> --- a/rust/kernel/mm/mmput_async.rs
> +++ b/rust/kernel/mm/mmput_async.rs
> @@ -10,7 +10,8 @@
>   use crate::{
>       bindings,
>       mm::MmWithUser,
> -    sync::aref::{ARef, AlwaysRefCounted},
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted},
>   };
>   use core::{ops::Deref, ptr::NonNull};
>   
> @@ -34,7 +35,7 @@ unsafe impl Send for MmWithUserAsync {}
>   unsafe impl Sync for MmWithUserAsync {}
>   
>   // SAFETY: By the type invariants, this type is always refcounted.
> -unsafe impl AlwaysRefCounted for MmWithUserAsync {
> +unsafe impl RefCounted for MmWithUserAsync {
>       #[inline]
>       fn inc_ref(&self) {
>           // SAFETY: The pointer is valid since self is a reference.
> @@ -48,6 +49,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<MmWithUserAsync>`
> +// from a `&MmWithUserAsync`.
> +unsafe impl AlwaysRefCounted for MmWithUserAsync {}
> +
>   // Make all `MmWithUser` methods available on `MmWithUserAsync`.
>   impl Deref for MmWithUserAsync {
>       type Target = MmWithUser;
> diff --git a/rust/kernel/opp.rs b/rust/kernel/opp.rs
> index a760fac287655..06fe2ca776a4f 100644
> --- a/rust/kernel/opp.rs
> +++ b/rust/kernel/opp.rs
> @@ -16,8 +16,8 @@
>       ffi::{c_char, c_ulong},
>       prelude::*,
>       str::CString,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::Opaque,
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, Opaque},
>   };
>   
>   #[cfg(CONFIG_CPU_FREQ)]
> @@ -1041,7 +1041,7 @@ unsafe impl Send for OPP {}
>   unsafe impl Sync for OPP {}
>   
>   /// SAFETY: The type invariants guarantee that [`OPP`] is always refcounted.
> -unsafe impl AlwaysRefCounted for OPP {
> +unsafe impl RefCounted for OPP {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference means that the refcount is nonzero.
>           unsafe { bindings::dev_pm_opp_get(self.0.get()) };
> @@ -1053,6 +1053,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<OPP>` from an
> +// `&OPP`.
> +unsafe impl AlwaysRefCounted for OPP {}
> +
>   impl OPP {
>       /// Creates an owned reference to a [`OPP`] from a valid pointer.
>       ///
> diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
> index fe30580331df9..b02edda11fcf6 100644
> --- a/rust/kernel/owned.rs
> +++ b/rust/kernel/owned.rs
> @@ -25,7 +25,7 @@
>   ///
>   /// Note: The underlying object is not required to provide internal reference counting, because it
>   /// represents a unique, owned reference. If reference counting (on the Rust side) is required,
> -/// [`AlwaysRefCounted`](crate::types::AlwaysRefCounted) should be implemented.
> +/// [`RefCounted`](crate::types::RefCounted) should be implemented.
>   ///
>   /// # Safety
>   ///
> diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
> index bea76ca9c3da5..9ee8f2bc6db9e 100644
> --- a/rust/kernel/pci.rs
> +++ b/rust/kernel/pci.rs
> @@ -19,6 +19,10 @@
>       },
>       prelude::*,
>       str::CStr,
> +    sync::aref::{
> +        AlwaysRefCounted,
> +        RefCounted, //
> +    },
>       types::Opaque,
>       ThisModule, //
>   };
> @@ -467,7 +471,7 @@ unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for Device<Ctx>
>   impl crate::dma::Device for Device<device::Core> {}
>   
>   // SAFETY: Instances of `Device` are always reference-counted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
> +unsafe impl RefCounted for Device {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>           unsafe { bindings::pci_dev_get(self.as_raw()) };
> @@ -479,6 +483,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl AlwaysRefCounted for Device {}
> +
>   impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
>       fn as_ref(&self) -> &device::Device<Ctx> {
>           // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> diff --git a/rust/kernel/pid_namespace.rs b/rust/kernel/pid_namespace.rs
> index 979a9718f153d..4f6a94540e33d 100644
> --- a/rust/kernel/pid_namespace.rs
> +++ b/rust/kernel/pid_namespace.rs
> @@ -7,7 +7,11 @@
>   //! C header: [`include/linux/pid_namespace.h`](srctree/include/linux/pid_namespace.h) and
>   //! [`include/linux/pid.h`](srctree/include/linux/pid.h)
>   
> -use crate::{bindings, sync::aref::AlwaysRefCounted, types::Opaque};
> +use crate::{
> +    bindings,
> +    sync::aref::RefCounted,
> +    types::{AlwaysRefCounted, Opaque},
> +};
>   use core::ptr;
>   
>   /// Wraps the kernel's `struct pid_namespace`. Thread safe.
> @@ -41,7 +45,7 @@ pub unsafe fn from_ptr<'a>(ptr: *const bindings::pid_namespace) -> &'a Self {
>   }
>   
>   // SAFETY: Instances of `PidNamespace` are always reference-counted.
> -unsafe impl AlwaysRefCounted for PidNamespace {
> +unsafe impl RefCounted for PidNamespace {
>       #[inline]
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -55,6 +59,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<PidNamespace>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<PidNamespace>` from
> +// a `&PidNamespace`.
> +unsafe impl AlwaysRefCounted for PidNamespace {}
> +
>   // SAFETY:
>   // - `PidNamespace::dec_ref` can be called from any thread.
>   // - It is okay to send ownership of `PidNamespace` across thread boundaries.
> diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
> index 35a5813ffb33f..139517c21961e 100644
> --- a/rust/kernel/platform.rs
> +++ b/rust/kernel/platform.rs
> @@ -13,6 +13,7 @@
>       irq::{self, IrqRequest},
>       of,
>       prelude::*,
> +    sync::aref::{AlwaysRefCounted, RefCounted},
>       types::Opaque,
>       ThisModule,
>   };
> @@ -490,7 +491,7 @@ pub fn optional_irq_by_name(&self, name: &CStr) -> Result<IrqRequest<'_>> {
>   impl crate::dma::Device for Device<device::Core> {}
>   
>   // SAFETY: Instances of `Device` are always reference-counted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Device {
> +unsafe impl RefCounted for Device {
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>           unsafe { bindings::get_device(self.as_ref().as_raw()) };
> @@ -502,6 +503,10 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Device>` from a
> +// `&Device`.
> +unsafe impl AlwaysRefCounted for Device {}
> +
>   impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
>       fn as_ref(&self) -> &device::Device<Ctx> {
>           // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
> diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs
> index e175aefe86151..61caddfd89619 100644
> --- a/rust/kernel/sync/aref.rs
> +++ b/rust/kernel/sync/aref.rs
> @@ -19,11 +19,9 @@
>   
>   use core::{marker::PhantomData, mem::ManuallyDrop, ops::Deref, ptr::NonNull};
>   
> -/// Types that are _always_ reference counted.
> +/// Types that are internally reference counted.
>   ///
>   /// It allows such types to define their own custom ref increment and decrement functions.
> -/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
> -/// [`ARef<T>`].
>   ///
>   /// This is usually implemented by wrappers to existing structures on the C side of the code. For
>   /// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
> @@ -40,9 +38,8 @@
>   /// at least until matching decrements are performed.
>   ///
>   /// Implementers must also ensure that all instances are reference-counted. (Otherwise they
> -/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
> -/// alive.)
> -pub unsafe trait AlwaysRefCounted {
> +/// won't be able to honour the requirement that [`RefCounted::inc_ref`] keep the object alive.)
> +pub unsafe trait RefCounted {
>       /// Increments the reference count on the object.
>       fn inc_ref(&self);
>   
> @@ -55,11 +52,27 @@ pub unsafe trait AlwaysRefCounted {
>       /// Callers must ensure that there was a previous matching increment to the reference count,
>       /// and that the object is no longer used after its reference count is decremented (as it may
>       /// result in the object being freed), unless the caller owns another increment on the refcount
> -    /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
> -    /// [`AlwaysRefCounted::dec_ref`] once).
> +    /// (e.g., it calls [`RefCounted::inc_ref`] twice, then calls [`RefCounted::dec_ref`] once).
>       unsafe fn dec_ref(obj: NonNull<Self>);
>   }
>   
> +/// Always reference-counted type.
> +///
> +/// It allows deriving a counted reference [`ARef<T>`] from a `&T`.
> +///
> +/// This provides some convenience, but it allows "escaping" borrow checks on `&T`. As it
> +/// complicates attempts to ensure that a reference to T is unique, it is optional to provide for
> +/// [`RefCounted`] types. See *Safety* below.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that no safety invariants are violated by upgrading an `&T` to an
> +/// [`ARef<T>`]. In particular that implies [`AlwaysRefCounted`] and [`crate::types::Ownable`]
> +/// cannot be implemented for the same type, as this would allow violating the uniqueness guarantee
> +/// of [`crate::types::Owned<T>`] by dereferencing it into an `&T` and obtaining an [`ARef`] from
> +/// that.
> +pub unsafe trait AlwaysRefCounted: RefCounted {}
> +
>   /// An owned reference to an always-reference-counted object.
>   ///
>   /// The object's reference count is automatically decremented when an instance of [`ARef`] is
> @@ -70,7 +83,7 @@ pub unsafe trait AlwaysRefCounted {
>   ///
>   /// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
>   /// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
> -pub struct ARef<T: AlwaysRefCounted> {
> +pub struct ARef<T: RefCounted> {
>       ptr: NonNull<T>,
>       _p: PhantomData<T>,
>   }
> @@ -79,16 +92,16 @@ pub struct ARef<T: AlwaysRefCounted> {
>   // it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
>   // `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
>   // mutable reference, for example, when the reference count reaches zero and `T` is dropped.
> -unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
> +unsafe impl<T: RefCounted + Sync + Send> Send for ARef<T> {}
>   
>   // SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
>   // because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
>   // it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
>   // `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
>   // example, when the reference count reaches zero and `T` is dropped.
> -unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
> +unsafe impl<T: RefCounted + Sync + Send> Sync for ARef<T> {}
>   
> -impl<T: AlwaysRefCounted> ARef<T> {
> +impl<T: RefCounted> ARef<T> {
>       /// Creates a new instance of [`ARef`].
>       ///
>       /// It takes over an increment of the reference count on the underlying object.
> @@ -117,12 +130,12 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
>       ///
>       /// ```
>       /// use core::ptr::NonNull;
> -    /// use kernel::sync::aref::{ARef, AlwaysRefCounted};
> +    /// use kernel::sync::aref::{ARef, RefCounted};
>       ///
>       /// struct Empty {}
>       ///
>       /// # // SAFETY: TODO.
> -    /// unsafe impl AlwaysRefCounted for Empty {
> +    /// unsafe impl RefCounted for Empty {
>       ///     fn inc_ref(&self) {}
>       ///     unsafe fn dec_ref(_obj: NonNull<Self>) {}
>       /// }
> @@ -140,7 +153,7 @@ pub fn into_raw(me: Self) -> NonNull<T> {
>       }
>   }
>   
> -impl<T: AlwaysRefCounted> Clone for ARef<T> {
> +impl<T: RefCounted> Clone for ARef<T> {
>       fn clone(&self) -> Self {
>           self.inc_ref();
>           // SAFETY: We just incremented the refcount above.
> @@ -148,7 +161,7 @@ fn clone(&self) -> Self {
>       }
>   }
>   
> -impl<T: AlwaysRefCounted> Deref for ARef<T> {
> +impl<T: RefCounted> Deref for ARef<T> {
>       type Target = T;
>   
>       fn deref(&self) -> &Self::Target {
> @@ -165,7 +178,7 @@ fn from(b: &T) -> Self {
>       }
>   }
>   
> -impl<T: AlwaysRefCounted> Drop for ARef<T> {
> +impl<T: RefCounted> Drop for ARef<T> {
>       fn drop(&mut self) {
>           // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
>           // decrement.
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 49fad6de06740..0a6e38d984560 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -9,8 +9,8 @@
>       ffi::{c_int, c_long, c_uint},
>       mm::MmWithUser,
>       pid_namespace::PidNamespace,
> -    sync::aref::ARef,
> -    types::{NotThreadSafe, Opaque},
> +    sync::aref::{AlwaysRefCounted, RefCounted},
> +    types::{ARef, NotThreadSafe, Opaque},
>   };
>   use core::{
>       cmp::{Eq, PartialEq},
> @@ -348,7 +348,7 @@ pub fn active_pid_ns(&self) -> Option<&PidNamespace> {
>   }
>   
>   // SAFETY: The type invariants guarantee that `Task` is always refcounted.
> -unsafe impl crate::sync::aref::AlwaysRefCounted for Task {
> +unsafe impl RefCounted for Task {
>       #[inline]
>       fn inc_ref(&self) {
>           // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> @@ -362,6 +362,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>       }
>   }
>   
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<Task>` from a
> +// `&Task`.
> +unsafe impl AlwaysRefCounted for Task {}
> +
>   impl Kuid {
>       /// Get the current euid.
>       #[inline]
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 4aec7b699269a..9b96aa2ebdb7e 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -18,7 +18,8 @@
>       },
>       sync::aref::{
>           ARef,
> -        AlwaysRefCounted, //
> +        AlwaysRefCounted,
> +        RefCounted, //
>       }, //
>   };
>   
> 

For Rust I2C subsystem:

Acked-by: Igor Korotin <igor.korotin.linux@gmail.com>

Thanks
Igor

Re: [PATCH v14 1/9] rust: types: Add Ownable/Owned types

[Danilo Krummrich]

On Wed Feb 4, 2026 at 12:56 PM CET, Andreas Hindborg wrote:
> From: Asahi Lina <lina+kernel@asahilina.net>
>
> By analogy to `AlwaysRefCounted` and `ARef`, an `Ownable` type is a
> (typically C FFI) type that *may* be owned by Rust, but need not be. Unlike
> `AlwaysRefCounted`, this mechanism expects the reference to be unique
> within Rust, and does not allow cloning.
>
> Conceptually, this is similar to a `KBox<T>`, except that it delegates
> resource management to the `T` instead of using a generic allocator.
>
> This change is a derived work based on work by Asahi Lina
> <lina+kernel@asahilina.net> [1] and Oliver Mangold <oliver.mangold@pm.me>.
>
> Link: https://lore.kernel.org/rust-for-linux/20250202-rust-page-v1-1-e3170d7fe55e@asahilina.net/ [1]
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Given the From: line above, this needs Lina's SoB.

This patch was also originally posted by Abdiel and Boqun and I think we should
account for this. I mentioned this in a couple of previous versions already,
e.g. in [1]. I think we should account for this.

[1] https://lore.kernel.org/all/cc28d048-5e0f-4f0e-b0f2-1b9e240f639b@kernel.org/

Re: [PATCH v14 2/9] rust: rename `AlwaysRefCounted` to `RefCounted`.

[Danilo Krummrich]

On Wed Feb 4, 2026 at 12:56 PM CET, Andreas Hindborg wrote:
>  rust/kernel/auxiliary.rs        |  7 +++++-
>  rust/kernel/device.rs           | 10 ++++++---
>  rust/kernel/device/property.rs  |  7 +++++-
>  rust/kernel/drm/device.rs       | 10 ++++++---
>  rust/kernel/drm/gem/mod.rs      |  8 ++++---
>  rust/kernel/pci.rs              | 10 ++++++++-
>  rust/kernel/platform.rs         |  7 +++++-

If you apply the kernel import style, those are

Acked-by: Danilo Krummrich <dakr@kernel.org>

However, I wonder if we want aref::* in the prelude instead.

Re: [PATCH v14 1/9] rust: types: Add Ownable/Owned types

[Andreas Hindborg]

"Danilo Krummrich" <dakr@kernel.org> writes:

> On Wed Feb 4, 2026 at 12:56 PM CET, Andreas Hindborg wrote:
>> From: Asahi Lina <lina+kernel@asahilina.net>
>>
>> By analogy to `AlwaysRefCounted` and `ARef`, an `Ownable` type is a
>> (typically C FFI) type that *may* be owned by Rust, but need not be. Unlike
>> `AlwaysRefCounted`, this mechanism expects the reference to be unique
>> within Rust, and does not allow cloning.
>>
>> Conceptually, this is similar to a `KBox<T>`, except that it delegates
>> resource management to the `T` instead of using a generic allocator.
>>
>> This change is a derived work based on work by Asahi Lina
>> <lina+kernel@asahilina.net> [1] and Oliver Mangold <oliver.mangold@pm.me>.
>>
>> Link: https://lore.kernel.org/rust-for-linux/20250202-rust-page-v1-1-e3170d7fe55e@asahilina.net/ [1]
>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>
> Given the From: line above, this needs Lina's SoB.
>
> This patch was also originally posted by Abdiel and Boqun and I think we should
> account for this. I mentioned this in a couple of previous versions already,
> e.g. in [1]. I think we should account for this.
>
> [1] https://lore.kernel.org/all/cc28d048-5e0f-4f0e-b0f2-1b9e240f639b@kernel.org/

I had a question about this in the cover letter.

It is my understanding that the SoB needs confirmation from the author
if the code was changed. I changed the code and did not want to bother
the original author, because it is my understanding they do not wish to
be contacted. I did not want to misrepresent the original author, and so
I did not change the "From:" line.

I want to be clear that I want to submit this patch the correct way,
whatever that is. I will happily take all the guidance I can get to find
the correct way to represent the work of the original author(s).

How would you prefer to account for the work by Abdiel and Boqun?

Please advise about SoB and I will correct the series.


Best regards,
Andreas Hindborg

Re: [PATCH v14 1/9] rust: types: Add Ownable/Owned types

[Danilo Krummrich]

On Wed Feb 4, 2026 at 5:06 PM CET, Andreas Hindborg wrote:
> It is my understanding that the SoB needs confirmation from the author
> if the code was changed. I changed the code and did not want to bother
> the original author, because it is my understanding they do not wish to
> be contacted. I did not want to misrepresent the original author, and so
> I did not change the "From:" line.

Frankly, I don't know what's the correct thing to do in this case; maybe the
correct thing is to just keep the SoB, but list all the changes that have been
made.

Technically, the same thing is common practice when maintainers (including
myself) apply (minor) changes to a patch when applying them to their tree.

> How would you prefer to account for the work by Abdiel and Boqun?

I mean, I don't have a preference and if I would have one, it wouldn't be
relevant. :) I just wanted to bring it up since the very first version was sent
by the two of them. So, I think we should just ask.

Re: [PATCH v14 1/9] rust: types: Add Ownable/Owned types

[Gary Guo]

On Wed Feb 4, 2026 at 4:46 PM GMT, Danilo Krummrich wrote:
> On Wed Feb 4, 2026 at 5:06 PM CET, Andreas Hindborg wrote:
>> It is my understanding that the SoB needs confirmation from the author
>> if the code was changed. I changed the code and did not want to bother
>> the original author, because it is my understanding they do not wish to
>> be contacted. I did not want to misrepresent the original author, and so
>> I did not change the "From:" line.
>
> Frankly, I don't know what's the correct thing to do in this case; maybe the
> correct thing is to just keep the SoB, but list all the changes that have been
> made.
>
> Technically, the same thing is common practice when maintainers (including
> myself) apply (minor) changes to a patch when applying them to their tree.
>
>> How would you prefer to account for the work by Abdiel and Boqun?
>
> I mean, I don't have a preference and if I would have one, it wouldn't be
> relevant. :) I just wanted to bring it up since the very first version was sent
> by the two of them. So, I think we should just ask.

It looks to me that they're independent works, there're very clear difference
between the two patches. Lina's patch closely follows the ARef design, and it
looks it also takes into account the dropck (by having `PhantomData`) and have
send/sync considerations.

Best,
Gary

Re: [PATCH v14 1/9] rust: types: Add Ownable/Owned types

[Gary Guo]

On Wed Feb 4, 2026 at 4:06 PM GMT, Andreas Hindborg wrote:
> "Danilo Krummrich" <dakr@kernel.org> writes:
>
>> On Wed Feb 4, 2026 at 12:56 PM CET, Andreas Hindborg wrote:
>>> From: Asahi Lina <lina+kernel@asahilina.net>
>>>
>>> By analogy to `AlwaysRefCounted` and `ARef`, an `Ownable` type is a
>>> (typically C FFI) type that *may* be owned by Rust, but need not be. Unlike
>>> `AlwaysRefCounted`, this mechanism expects the reference to be unique
>>> within Rust, and does not allow cloning.
>>>
>>> Conceptually, this is similar to a `KBox<T>`, except that it delegates
>>> resource management to the `T` instead of using a generic allocator.
>>>
>>> This change is a derived work based on work by Asahi Lina
>>> <lina+kernel@asahilina.net> [1] and Oliver Mangold <oliver.mangold@pm.me>.
>>>
>>> Link: https://lore.kernel.org/rust-for-linux/20250202-rust-page-v1-1-e3170d7fe55e@asahilina.net/ [1]
>>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>>
>> Given the From: line above, this needs Lina's SoB.
>>
>> This patch was also originally posted by Abdiel and Boqun and I think we should
>> account for this. I mentioned this in a couple of previous versions already,
>> e.g. in [1]. I think we should account for this.
>>
>> [1] https://lore.kernel.org/all/cc28d048-5e0f-4f0e-b0f2-1b9e240f639b@kernel.org/
>
> I had a question about this in the cover letter.
>
> It is my understanding that the SoB needs confirmation from the author
> if the code was changed. I changed the code and did not want to bother
> the original author, because it is my understanding they do not wish to
> be contacted. I did not want to misrepresent the original author, and so
> I did not change the "From:" line.

Looks like your didn't make a very substantial change (just OwnableMut ->
Unpin)? So mentioning the change in commit message should be sufficient.

If it's very substantial I would change author to be you and add the original
author as Co-developed-by instead.

Best,
Gary

>
> I want to be clear that I want to submit this patch the correct way,
> whatever that is. I will happily take all the guidance I can get to find
> the correct way to represent the work of the original author(s).
>
> How would you prefer to account for the work by Abdiel and Boqun?
>
> Please advise about SoB and I will correct the series.
>
>
> Best regards,
> Andreas Hindborg

Re: [PATCH v14 2/9] rust: rename `AlwaysRefCounted` to `RefCounted`.

[Viresh Kumar]

On 04-02-26, 12:56, Andreas Hindborg wrote:
> diff --git a/rust/kernel/opp.rs b/rust/kernel/opp.rs
> index a760fac287655..06fe2ca776a4f 100644
> --- a/rust/kernel/opp.rs
> +++ b/rust/kernel/opp.rs
> @@ -16,8 +16,8 @@
>      ffi::{c_char, c_ulong},
>      prelude::*,
>      str::CString,
> -    sync::aref::{ARef, AlwaysRefCounted},
> -    types::Opaque,
> +    sync::aref::RefCounted,
> +    types::{ARef, AlwaysRefCounted, Opaque},
>  };
>  
>  #[cfg(CONFIG_CPU_FREQ)]
> @@ -1041,7 +1041,7 @@ unsafe impl Send for OPP {}
>  unsafe impl Sync for OPP {}
>  
>  /// SAFETY: The type invariants guarantee that [`OPP`] is always refcounted.
> -unsafe impl AlwaysRefCounted for OPP {
> +unsafe impl RefCounted for OPP {
>      fn inc_ref(&self) {
>          // SAFETY: The existence of a shared reference means that the refcount is nonzero.
>          unsafe { bindings::dev_pm_opp_get(self.0.get()) };
> @@ -1053,6 +1053,10 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
>      }
>  }
>  
> +// SAFETY: We do not implement `Ownable`, thus it is okay to obtain an `ARef<OPP>` from an
> +// `&OPP`.
> +unsafe impl AlwaysRefCounted for OPP {}
> +
>  impl OPP {
>      /// Creates an owned reference to a [`OPP`] from a valid pointer.
>      ///

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

-- 
viresh

Re: [PATCH v14 1/9] rust: types: Add Ownable/Owned types

[Gary Guo]

On Wed Feb 4, 2026 at 11:56 AM GMT, Andreas Hindborg wrote:
> From: Asahi Lina <lina+kernel@asahilina.net>
>
> By analogy to `AlwaysRefCounted` and `ARef`, an `Ownable` type is a
> (typically C FFI) type that *may* be owned by Rust, but need not be. Unlike
> `AlwaysRefCounted`, this mechanism expects the reference to be unique
> within Rust, and does not allow cloning.
>
> Conceptually, this is similar to a `KBox<T>`, except that it delegates
> resource management to the `T` instead of using a generic allocator.
>
> This change is a derived work based on work by Asahi Lina
> <lina+kernel@asahilina.net> [1] and Oliver Mangold <oliver.mangold@pm.me>.
>
> Link: https://lore.kernel.org/rust-for-linux/20250202-rust-page-v1-1-e3170d7fe55e@asahilina.net/ [1]
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> ---
>  rust/kernel/lib.rs       |   1 +
>  rust/kernel/owned.rs     | 196 +++++++++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/sync/aref.rs |   5 ++
>  rust/kernel/types.rs     |  11 ++-
>  4 files changed, 212 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index f812cf1200428..96a3fadc3377a 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -119,6 +119,7 @@
>  pub mod of;
>  #[cfg(CONFIG_PM_OPP)]
>  pub mod opp;
> +pub mod owned;
>  pub mod page;
>  #[cfg(CONFIG_PCI)]
>  pub mod pci;
> diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs
> new file mode 100644
> index 0000000000000..fe30580331df9
> --- /dev/null
> +++ b/rust/kernel/owned.rs
> <snip>
> +
> +    /// Get a pinned mutable reference to the data owned by this `Owned<T>`.
> +    pub fn get_pin_mut(&mut self) -> Pin<&mut T> {
> +        // SAFETY: The type invariants guarantee that the object is valid, and that we can safely
> +        // return a mutable reference to it.
> +        let unpinned = unsafe { self.ptr.as_mut() };
> +
> +        // SAFETY: We never hand out unpinned mutable references to the data in
> +        // `Self`, unless the contained type is `Unpin`.
> +        unsafe { Pin::new_unchecked(unpinned) }
> +    }

Probably should be name `as_pin_mut` instead.

With name changed and SOB fixed:

Reviewed-by: Gary Guo <gary@garyguo.net>

Best,
Gary

> +}
> +
> +// SAFETY: It is safe to send an [`Owned<T>`] to another thread when the underlying `T` is [`Send`],
> +// because of the ownership invariant. Sending an [`Owned<T>`] is equivalent to sending the `T`.
> +unsafe impl<T: Ownable + Send> Send for Owned<T> {}
> +
> +// SAFETY: It is safe to send [`&Owned<T>`] to another thread when the underlying `T` is [`Sync`],
> +// because of the ownership invariant. Sending an [`&Owned<T>`] is equivalent to sending the `&T`.
> +unsafe impl<T: Ownable + Sync> Sync for Owned<T> {}
> +

Re: [PATCH v14 2/9] rust: rename `AlwaysRefCounted` to `RefCounted`.

[Gary Guo]

On Wed Feb 4, 2026 at 11:56 AM GMT, Andreas Hindborg wrote:
> From: Oliver Mangold <oliver.mangold@pm.me>
> 
> There are types where it may both be reference counted in some cases and
> owned in others. In such cases, obtaining `ARef<T>` from `&T` would be
> unsound as it allows creation of `ARef<T>` copy from `&Owned<T>`.
> 
> Therefore, we split `AlwaysRefCounted` into `RefCounted` (which `ARef<T>`
> would require) and a marker trait to indicate that the type is always
> reference counted (and not `Ownable`) so the `&T` -> `ARef<T>` conversion
> is possible.
> 
> - Rename `AlwaysRefCounted` to `RefCounted`.
> - Add a new unsafe trait `AlwaysRefCounted`.
> - Implement the new trait `AlwaysRefCounted` for the newly renamed
>   `RefCounted` implementations. This leaves functionality of existing
>   implementers of `AlwaysRefCounted` intact.
> 
> Original patch by Oliver Mangold <oliver.mangold@pm.me> [1].
> 
> Link: https://lore.kernel.org/r/20251117-unique-ref-v13-2-b5b243df1250@pm.me [1]
> Suggested-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

I think you also need to update the `AlwaysRefCounted` reference mentioned in
the `Owned` patch too? (Or perhaps this patch should be moved before `Owned`
instead?)

With that fixed:

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/auxiliary.rs        |  7 +++++-
>  rust/kernel/block/mq/request.rs | 15 +++++++------
>  rust/kernel/cred.rs             | 13 ++++++++++--
>  rust/kernel/device.rs           | 10 ++++++---
>  rust/kernel/device/property.rs  |  7 +++++-
>  rust/kernel/drm/device.rs       | 10 ++++++---
>  rust/kernel/drm/gem/mod.rs      |  8 ++++---
>  rust/kernel/fs/file.rs          | 16 ++++++++++----
>  rust/kernel/i2c.rs              | 16 +++++++++-----
>  rust/kernel/mm.rs               | 15 +++++++++----
>  rust/kernel/mm/mmput_async.rs   |  9 ++++++--
>  rust/kernel/opp.rs              | 10 ++++++---
>  rust/kernel/owned.rs            |  2 +-
>  rust/kernel/pci.rs              | 10 ++++++++-
>  rust/kernel/pid_namespace.rs    | 12 +++++++++--
>  rust/kernel/platform.rs         |  7 +++++-
>  rust/kernel/sync/aref.rs        | 47 ++++++++++++++++++++++++++---------------
>  rust/kernel/task.rs             | 10 ++++++---
>  rust/kernel/types.rs            |  3 ++-
>  19 files changed, 164 insertions(+), 63 deletions(-)

Re: [PATCH v14 2/9] rust: rename `AlwaysRefCounted` to `RefCounted`.

[Gary Guo]

On Thu Feb 5, 2026 at 1:31 PM GMT, Gary Guo wrote:
> On Wed Feb 4, 2026 at 11:56 AM GMT, Andreas Hindborg wrote:
>> From: Oliver Mangold <oliver.mangold@pm.me>
>> 
>> There are types where it may both be reference counted in some cases and
>> owned in others. In such cases, obtaining `ARef<T>` from `&T` would be
>> unsound as it allows creation of `ARef<T>` copy from `&Owned<T>`.
>> 
>> Therefore, we split `AlwaysRefCounted` into `RefCounted` (which `ARef<T>`
>> would require) and a marker trait to indicate that the type is always
>> reference counted (and not `Ownable`) so the `&T` -> `ARef<T>` conversion
>> is possible.
>> 
>> - Rename `AlwaysRefCounted` to `RefCounted`.
>> - Add a new unsafe trait `AlwaysRefCounted`.
>> - Implement the new trait `AlwaysRefCounted` for the newly renamed
>>   `RefCounted` implementations. This leaves functionality of existing
>>   implementers of `AlwaysRefCounted` intact.
>> 
>> Original patch by Oliver Mangold <oliver.mangold@pm.me> [1].
>> 
>> Link: https://lore.kernel.org/r/20251117-unique-ref-v13-2-b5b243df1250@pm.me [1]
>> Suggested-by: Alice Ryhl <aliceryhl@google.com>
>> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>
> I think you also need to update the `AlwaysRefCounted` reference mentioned in
> the `Owned` patch too? (Or perhaps this patch should be moved before `Owned`
> instead?)

Actually I re-read the comment in first patch, the text indeed should refer to
`AlwaysRefCounted`. Please disregard this comment.

Best,
Gary

>
> With that fixed:
>
> Reviewed-by: Gary Guo <gary@garyguo.net>
>
>> ---
>>  rust/kernel/auxiliary.rs        |  7 +++++-
>>  rust/kernel/block/mq/request.rs | 15 +++++++------
>>  rust/kernel/cred.rs             | 13 ++++++++++--
>>  rust/kernel/device.rs           | 10 ++++++---
>>  rust/kernel/device/property.rs  |  7 +++++-
>>  rust/kernel/drm/device.rs       | 10 ++++++---
>>  rust/kernel/drm/gem/mod.rs      |  8 ++++---
>>  rust/kernel/fs/file.rs          | 16 ++++++++++----
>>  rust/kernel/i2c.rs              | 16 +++++++++-----
>>  rust/kernel/mm.rs               | 15 +++++++++----
>>  rust/kernel/mm/mmput_async.rs   |  9 ++++++--
>>  rust/kernel/opp.rs              | 10 ++++++---
>>  rust/kernel/owned.rs            |  2 +-
>>  rust/kernel/pci.rs              | 10 ++++++++-
>>  rust/kernel/pid_namespace.rs    | 12 +++++++++--
>>  rust/kernel/platform.rs         |  7 +++++-
>>  rust/kernel/sync/aref.rs        | 47 ++++++++++++++++++++++++++---------------
>>  rust/kernel/task.rs             | 10 ++++++---
>>  rust/kernel/types.rs            |  3 ++-
>>  19 files changed, 164 insertions(+), 63 deletions(-)