Re: [PATCH v13 1/4] rust: types: Add Ownable/Owned types

[Andreas Hindborg <a.hindborg@kernel.org>]

Hi Daniel,

I'll send the next iteration of this series.

Daniel Almeida <daniel.almeida@collabora.com> writes:

> Hi Oliver,
>
>> On 17 Nov 2025, at 07:07, Oliver Mangold <oliver.mangold@pm.me> 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.
>> 
>> [ om:
>>  - Split code into separate file and `pub use` it from types.rs.
>>  - Make from_raw() and into_raw() public.
>>  - Remove OwnableMut, and make DerefMut dependent on Unpin instead.
>>  - Usage example/doctest for Ownable/Owned.
>>  - Fixes to documentation and commit message.
>> ]
>> 
>> Link: https://lore.kernel.org/all/20250202-rust-page-v1-1-e3170d7fe55e@asahilina.net/
>> Signed-off-by: Asahi Lina <lina+kernel@asahilina.net>
>> Co-developed-by: Oliver Mangold <oliver.mangold@pm.me>
>> Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>
>> Co-developed-by: Andreas Hindborg <a.hindborg@kernel.org>
>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
>> ---
>> rust/kernel/lib.rs       |   1 +
>> rust/kernel/owned.rs     | 195 +++++++++++++++++++++++++++++++++++++++++++++++
>> rust/kernel/sync/aref.rs |   5 ++
>> rust/kernel/types.rs     |   2 +
>> 4 files changed, 203 insertions(+)
>> 
>> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
>> index 3dd7bebe7888..e0ee04330dd0 100644
>> --- a/rust/kernel/lib.rs
>> +++ b/rust/kernel/lib.rs
>> @@ -112,6 +112,7 @@
>> 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 000000000000..a2cdd2cb8a10
>> --- /dev/null
>> +++ b/rust/kernel/owned.rs
>> @@ -0,0 +1,195 @@
>> +// 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,
>> +};
>> +
>> +/// Type allocated and destroyed on the C side, but owned by Rust.
>> +///
>> +/// 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 {
>> +/// }
>
> nit: this can be simply:
>
> struct Foo;

Got it.

>
>> +///
>> +/// impl Foo {
>> +///     fn new() -> Result<Owned<Self>, AllocError> {
>> +///         // 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: What out `release()` function does is safe of any valid `Self`.
>> +/// unsafe impl Ownable for Foo {
>> +///     unsafe fn release(this: NonNull<Self>) {
>> +///         // The `Foo` will be dropped when `KBox` goes out of scope.
>> +///         // 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.
>> +///         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>);
>> +}
>> +
>> +/// An owned 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>,
>> +}
>> +
>> +// 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> Owned<T> {
>
> Can you make sure that impl Owned<T> follows the struct declaration?
>
> IOW: please move the Send and Sync impls to be after the impl above.

I don't really see the point, but moving them is no problem, so let's do that.

>
>> +    /// Creates a new instance of [`Owned`].
>> +    ///
>> +    /// It 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).
>> +    /// - No other Rust references to the underlying object exist. This implies that the underlying
>> +    ///   object is not accessed through `ptr` anymore after the function call (at least until the
>> +    ///   the `Self<T>` is dropped.
>
> It looks like this can be written more succinctly as:
>
> "This implies that the underlying object is not accessed through `ptr` anymore until `Self<T>` is dropped."

I'll rephrase with that text.

>
>> +    /// - The C code follows the usual shared reference requirements. That is, the kernel will never
>> +    ///   mutate or free the underlying object (excluding interior mutability that follows the usual
>> +    ///   rules) while Rust owns it.
>> +    /// - In case `T` implements [`Unpin`] the previous requirement is extended from shared to
>> +    ///   mutable reference requirements. That is, the kernel will not mutate or free the underlying
>> +    ///   object and is okay with it being modified by Rust code.
>> +    pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
>> +        Self {
>> +            ptr,
>> +        }
>> +    }
>> +
>> +    /// Consumes the [`Owned`], returning a raw pointer.
>> +    ///
>> +    /// This function does not actually relinquish ownership of the object. After calling this
>> +    /// function, the caller is responsible for ownership previously managed
>> +    /// by the [`Owned`].
>> +    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) }
>> +    }
>> +}
>> +
>> +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 0d24a0432015..e175aefe8615 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 dc0a02f5c3cf..7bc07c38cd6c 100644
>> --- a/rust/kernel/types.rs
>> +++ b/rust/kernel/types.rs
>> @@ -11,6 +11,8 @@
>> };
>> use pin_init::{PinInit, Wrapper, Zeroable};
>> 
>> +pub use crate::owned::{Ownable, Owned};
>> +
>> pub use crate::sync::aref::{ARef, AlwaysRefCounted};
>> 
>> /// Used to transfer ownership to and from foreign (non-Rust) languages.
>> 
>> -- 
>> 2.51.2
>> 
>> 
>> 
>
> With the changes above,
>
> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>

Thanks!