Re: [PATCH v3 01/25] rust: alloc: add `Allocator` trait

Alice Ryhl
2024-08-01T08:19:55Z

On Thu, Aug 1, 2024 at 2:07 AM Danilo Krummrich <dakr@kernel.org> wrote:
>
> Add a kernel specific `Allocator` trait, that in contrast to the one in
> Rust's core library doesn't require unstable features and supports GFP
> flags.
>
> Subsequent patches add the following trait implementors: `Kmalloc`,
> `Vmalloc` and `KVmalloc`.
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

It could be nice to mention that the design without `self` parameters
is necessary for compatibility with #[derive(SmartPointer)].

> rust/kernel/alloc.rs | 73 ++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 73 insertions(+)
>
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index 1966bd407017..b79dd2c49277 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -11,6 +11,7 @@
> /// Indicates an allocation error.
> #[derive(Copy, Clone, PartialEq, Eq, Debug)]
> pub struct AllocError;
> +use core::{alloc::Layout, ptr::NonNull};
>
> /// Flags to be used when allocating memory.
> ///
> @@ -86,3 +87,75 @@ pub mod flags {
> /// small allocations.
> pub const GFP_NOWAIT: Flags = Flags(bindings::GFP_NOWAIT);
> }
> +
> +/// The kernel's [`Allocator`] trait.
> +///
> +/// An implementation of [`Allocator`] can allocate, re-allocate and free memory buffer described
> +/// via [`Layout`].
> +///
> +/// [`Allocator`] is designed to be implemented as a ZST; [`Allocator`] functions do not operate on
> +/// an object instance.
> +///
> +/// # Safety
> +///
> +/// Memory returned from an allocator must point to a valid memory buffer and remain valid until
> +/// it is explicitly freed.
> +///
> +/// Any pointer to a memory buffer which is currently allocated must be valid to be passed to any
> +/// other [`Allocator`] function. The same applies for a NULL pointer.
> +///
> +/// If `realloc` is called with:
> +/// - a size of zero, the given memory allocation, if any, must be freed
> +/// - a NULL pointer, a new memory allocation must be created
> +pub unsafe trait Allocator {
> + /// Allocate memory based on `layout` and `flags`.
> + ///
> + /// On success, returns a buffer represented as `NonNull<[u8]>` that satisfies the layout
> + /// constraints (i.e. minimum size and alignment as specified by `layout`).
> + ///
> + /// This function is equivalent to `realloc` when called with a NULL pointer.
> + fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
> + // SAFETY: Passing a NULL pointer to `realloc` is valid by it's safety requirements and asks
> + // for a new memory allocation.
> + unsafe { Self::realloc(None, layout, flags) }
> + }
> +
> + /// Re-allocate an existing memory allocation to satisfy the requested `layout`. If the
> + /// requested size is zero, `realloc` behaves equivalent to `free`.
> + ///
> + /// If the requested size is larger than the size of the existing allocation, a successful call
> + /// to `realloc` guarantees that the new or grown buffer has at least `Layout::size` bytes, but
> + /// may also be larger.
> + ///
> + /// If the requested size is smaller than the size of the existing allocation, `realloc` may or
> + /// may not shrink the buffer; this is implementation specific to the allocator.
> + ///
> + /// On allocation failure, the existing buffer, if any, remains valid.
> + ///
> + /// The buffer is represented as `NonNull<[u8]>`.
> + ///
> + /// # Safety
> + ///
> + /// `ptr` must point to an existing and valid memory allocation created by this allocator
> + /// instance.
> + ///
> + /// Additionally, `ptr` is allowed to be a NULL pointer; in this case a new memory allocation is
> + /// created.
> + unsafe fn realloc(
> + ptr: Option<NonNull<u8>>,
> + layout: Layout,
> + flags: Flags,
> + ) -> Result<NonNull<[u8]>, AllocError>;

Is it intentional that this allows you to change the alignment of an
allocation? If so, that could use a note in the docs.

> + /// Free an existing memory allocation.
> + ///
> + /// # Safety
> + ///
> + /// `ptr` must point to an existing and valid memory allocation created by this `Allocator`
> + /// instance.
> + unsafe fn free(ptr: NonNull<u8>) {
> + // SAFETY: `ptr` is guaranteed to be previously allocated with this `Allocator` or NULL.
> + // Calling `realloc` with a buffer size of zero, frees the buffer `ptr` points to.
> + let _ = unsafe { Self::realloc(Some(ptr), Layout::new::<()>(), Flags(0)) };
> + }

At the very least, the provided implementation of `free` changes the
alignment when it calls `realloc`.

Alice