Re: [PATCH v3] rust: mm: add abstractions for mm_struct and vm_area_struct

[Benno Lossin <benno.lossin@proton.me>]

On 01.08.24 14:58, Alice Ryhl wrote:
> diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
> new file mode 100644
> index 000000000000..ed2db893fb79
> --- /dev/null
> +++ b/rust/kernel/mm.rs
> @@ -0,0 +1,337 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2024 Google LLC.
> +
> +//! Memory management.
> +//!
> +//! C header: [`include/linux/mm.h`](../../../../include/linux/mm.h)
> +
> +use crate::{
> +    bindings,
> +    types::{ARef, AlwaysRefCounted, Opaque},
> +};
> +
> +use core::{
> +    ops::Deref,
> +    ptr::{self, NonNull},
> +};
> +
> +pub mod virt;
> +
> +/// A wrapper for the kernel's `struct mm_struct`.
> +///
> +/// Since `mm_users` may be zero, the associated address space may not exist anymore. You must use

Can it also be the case that the space never existed to begin with? Then
I would write "the associated address space may not exist."

Also I think it makes more sense to use "You can use [`mmget_not_zero`]
to be able to access the address space." instead of the second sentence.

> +/// [`mmget_not_zero`] before accessing the address space.
> +///
> +/// The `ARef<Mm>` smart pointer holds an `mmgrab` refcount. Its destructor may sleep.
> +///
> +/// # Invariants
> +///
> +/// Values of this type are always refcounted.

Would be good to record the refcount used in the invariant.

> +///
> +/// [`mmget_not_zero`]: Mm::mmget_not_zero
> +pub struct Mm {
> +    mm: Opaque<bindings::mm_struct>,
> +}
> +
> +// SAFETY: It is safe to call `mmdrop` on another thread than where `mmgrab` was called.
> +unsafe impl Send for Mm {}
> +// SAFETY: All methods on `Mm` can be called in parallel from several threads.
> +unsafe impl Sync for Mm {}
> +
> +// SAFETY: By the type invariants, this type is always refcounted.
> +unsafe impl AlwaysRefCounted for Mm {
> +    fn inc_ref(&self) {
> +        // SAFETY: The pointer is valid since self is a reference.
> +        unsafe { bindings::mmgrab(self.as_raw()) };
> +    }
> +
> +    unsafe fn dec_ref(obj: NonNull<Self>) {
> +        // SAFETY: The caller is giving up their refcount.
> +        unsafe { bindings::mmdrop(obj.cast().as_ptr()) };
> +    }
> +}
> +
> +/// A wrapper for the kernel's `struct mm_struct`.
> +///
> +/// This type is used only when `mm_users` is known to be non-zero at compile-time. It can be used

I find the "This type is used only when" a bit weird, what about "Like
an [`Mm`], but with non-zero `mm_users`."?

> +/// to access the associated address space.
> +///
> +/// The `ARef<MmWithUser>` smart pointer holds an `mmget` refcount. Its destructor may sleep.
> +///
> +/// # Invariants
> +///
> +/// Values of this type are always refcounted. The value of `mm_users` is non-zero.
> +#[repr(transparent)]
> +pub struct MmWithUser {
> +    mm: Mm,
> +}
> +
> +// SAFETY: It is safe to call `mmput` on another thread than where `mmget` was called.
> +unsafe impl Send for MmWithUser {}
> +// SAFETY: All methods on `MmWithUser` can be called in parallel from several threads.
> +unsafe impl Sync for MmWithUser {}
> +
> +// SAFETY: By the type invariants, this type is always refcounted.
> +unsafe impl AlwaysRefCounted for MmWithUser {
> +    fn inc_ref(&self) {
> +        // SAFETY: The pointer is valid since self is a reference.
> +        unsafe { bindings::mmget(self.as_raw()) };
> +    }
> +
> +    unsafe fn dec_ref(obj: NonNull<Self>) {
> +        // SAFETY: The caller is giving up their refcount.
> +        unsafe { bindings::mmput(obj.cast().as_ptr()) };
> +    }
> +}
> +
> +// Make all `Mm` methods available on `MmWithUser`.
> +impl Deref for MmWithUser {
> +    type Target = Mm;
> +
> +    #[inline]
> +    fn deref(&self) -> &Mm {
> +        &self.mm
> +    }
> +}
> +
> +/// A wrapper for the kernel's `struct mm_struct`.
> +///
> +/// This type is identical to `MmWithUser` except that it uses `mmput_async` when dropping a
> +/// refcount. This means that the destructor of `ARef<MmWithUserAsync>` is safe to call in atomic
> +/// context.

Missing Invariants.

> +#[repr(transparent)]
> +pub struct MmWithUserAsync {
> +    mm: MmWithUser,
> +}
> +
> +// SAFETY: It is safe to call `mmput_async` on another thread than where `mmget` was called.
> +unsafe impl Send for MmWithUserAsync {}
> +// SAFETY: All methods on `MmWithUserAsync` can be called in parallel from several threads.
> +unsafe impl Sync for MmWithUserAsync {}
> +
> +// SAFETY: By the type invariants, this type is always refcounted.
> +unsafe impl AlwaysRefCounted for MmWithUserAsync {
> +    fn inc_ref(&self) {
> +        // SAFETY: The pointer is valid since self is a reference.
> +        unsafe { bindings::mmget(self.as_raw()) };
> +    }
> +
> +    unsafe fn dec_ref(obj: NonNull<Self>) {
> +        // SAFETY: The caller is giving up their refcount.
> +        unsafe { bindings::mmput_async(obj.cast().as_ptr()) };
> +    }
> +}
> +
> +// Make all `MmWithUser` methods available on `MmWithUserAsync`.
> +impl Deref for MmWithUserAsync {
> +    type Target = MmWithUser;
> +
> +    #[inline]
> +    fn deref(&self) -> &MmWithUser {
> +        &self.mm
> +    }
> +}
> +
> +// These methods are safe to call even if `mm_users` is zero.
> +impl Mm {
> +    /// Call `mmgrab` on `current.mm`.
> +    #[inline]
> +    pub fn mmgrab_current() -> Option<ARef<Mm>> {
> +        // SAFETY: It's safe to get the `mm` field from current.
> +        let mm = unsafe {
> +            let current = bindings::get_current();
> +            (*current).mm
> +        };
> +
> +        let mm = NonNull::new(mm)?;
> +
> +        // SAFETY: We just checked that `mm` is not null.
> +        unsafe { bindings::mmgrab(mm.as_ptr()) };
> +
> +        // SAFETY: We just created an `mmgrab` refcount. Layouts are compatible due to
> +        // repr(transparent).
> +        Some(unsafe { ARef::from_raw(mm.cast()) })
> +    }
> +
> +    /// Returns a raw pointer to the inner `mm_struct`.
> +    #[inline]
> +    pub fn as_raw(&self) -> *mut bindings::mm_struct {
> +        self.mm.get()
> +    }
> +
> +    /// Obtain a reference from a raw pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that `ptr` points at an `mm_struct`, and that it is not deallocated
> +    /// during the lifetime 'a.
> +    #[inline]
> +    pub unsafe fn from_raw_mm<'a>(ptr: *const bindings::mm_struct) -> &'a Mm {

Why not just `from_raw`?

---
Cheers,
Benno

> +        // SAFETY: Caller promises that the pointer is valid for 'a. Layouts are compatible due to
> +        // repr(transparent).
> +        unsafe { &*ptr.cast() }
> +    }