Re: [PATCH v5 01/26] rust: alloc: add `Allocator` trait

Benno Lossin
2024-08-14T16:13:18Z

On 12.08.24 20:22, Danilo Krummrich 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`.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
> rust/kernel/alloc.rs | 81 ++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 81 insertions(+)
>
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index 1966bd407017..194745498a75 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 +87,83 @@ 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.
> +///
> +/// In order to be able to support `#[derive(SmartPointer)]` later on, we need to avoid a design
> +/// that requires an `Allocator` to be instantiated, hence its functions must not contain any kind
> +/// of `self` parameter.
> +///
> +/// # Safety
> +///
> +/// Memory returned from an allocator must point to a valid memory buffer and remain valid until
> +/// it is explicitly freed.

I wouldn't say that an allocator "returns memory", and in general I
don't think the structure of the safety comment here is nice, how about
the following: we put "Implementers must ensure that all trait functions
abide by the guarantees documented in the `# Guarantees` sections."...

> +///
> +/// Any pointer to a memory buffer which is currently allocated must be valid to be passed to any
> +/// other [`Allocator`] function of the same type.
> +///
> +/// If `realloc` is called with:
> +/// - a size of zero, the given memory allocation, if any, must be freed
> +/// - `None`, 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 `None`.

... Then we can add this here:

/// # Guarantees
///
/// When the return value is `Ok(ptr)`, then `ptr` is
/// - valid for writes (and reads after the memory has been initialized) for `layout.size()` bytes,
/// until it is passed to [`Allocator::free`] or [`Allocator::realloc`],
/// - aligned to `layout.align()`,
/// - is valid for reads, if `flags.contains(flags::__GFP_ZERO)`,

Do we need to handle other flags?
Also IIRC the memory given to us by C is considered initialized by Rust
(though it has a non-deterministic value), so we might have an
unconditional "valid for reads". Am I correct?

> + fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
> + // SAFETY: Passing `None` 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`.

I don't think we want to include the second sentence in the short
description of this function, please add an empty line in between.

> + ///
> + /// 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
> + ///
> + /// If `ptr = Some(p)`, then `p` must point to an existing and valid memory allocation created

I don't like the single `=` (I might have written it in haste myself),
how about `==` or we use if-let syntax?

> + /// by this allocator. The alignment encoded in `layout` must be smaller than or equal to the
> + /// alignment requested in the previous `alloc` or `realloc` call of the same allocation.
> + ///
> + /// Additionally, `ptr` is allowed to be `None`; in this case a new memory allocation is
> + /// created.

This Safety section does not talk about the case `layout.size() == 0`,
but it should have the same requirement as `free()`.

Also add a `# Guarantees` section here:

/// # Guarantees
///
/// This function has the same guarantees as [`Allocator::alloc`]. When `ptr == Some(p)`, then it
/// additionally has the following:
/// - when `Ok(ret_ptr)` is the return value, then
/// `ret_ptr[0..min(layout.size(), old_size)] == p[0..min(layout.size(), old_size)]`, where
/// `old_size` is the size of the allocation that `p` points at.
/// - when the return value is `Err(AllocError)`, then `p` is still valid.

---
Cheers,
Benno

> + unsafe fn realloc(
> + ptr: Option<NonNull<u8>>,
> + layout: Layout,
> + flags: Flags,
> + ) -> Result<NonNull<[u8]>, AllocError>;

> +
> + /// Free an existing memory allocation.
> + ///
> + /// # Safety
> + ///
> + /// `ptr` must point to an existing and valid memory allocation created by this `Allocator` and
> + /// must not be a dangling pointer.
> + ///
> + /// The memory allocation at `ptr` must never again be read from or written to.
> + unsafe fn free(ptr: NonNull<u8>) {
> + // SAFETY: The caller guarantees that `ptr` points at a valid allocation created by this
> + // allocator. We are passing a `Layout` with the smallest possible alignment, so it is
> + // smaller than or equal to the alignment previously used with this allocation.
> + let _ = unsafe { Self::realloc(Some(ptr), Layout::new::<()>(), Flags(0)) };
> + }
> +}
> --
> 2.45.2
>