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

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Message-ID: <CAH5fLgj5xf4QdDU7yWrqUjcmJw4Rqe0-UzesHLhfQRvPqAv8eA@mail.gmail.com>
Date: Thu, 1 Aug 2024 10:19:41 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com, 
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, 
	benno.lossin@...ton.me, a.hindborg@...sung.com, akpm@...ux-foundation.org, 
	daniel.almeida@...labora.com, faith.ekstrand@...labora.com, 
	boris.brezillon@...labora.com, lina@...hilina.net, mcanal@...lia.com, 
	zhiw@...dia.com, acurrid@...dia.com, cjia@...dia.com, jhubbard@...dia.com, 
	airlied@...hat.com, ajanulgu@...hat.com, lyude@...hat.com, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH v3 01/25] rust: alloc: add `Allocator` trait

On Thu, Aug 1, 2024 at 2:07 AM Danilo Krummrich <dakr@...nel.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@...nel.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