Re: [PATCH 01/20] rust: alloc: add `Allocator` trait

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

On 04.07.24 19:06, 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`.
> 
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> ---
>  rust/kernel/alloc.rs | 73 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 73 insertions(+)
> 
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index 531b5e471cb1..462e00982510 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, ptr::NonNull};
> 
>  /// Flags to be used when allocating memory.
>  ///
> @@ -71,3 +72,75 @@ 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 on ZSTs; its safety requirements to not allow for
> +/// keeping a state throughout an instance.

Why do the functions take `&self` if it is forbidden to have state? I
would remove the receiver in that case.

> +///
> +/// # Safety
> +///
> +/// Memory returned from an allocator must point to a valid memory buffer and remain valid until
> +/// its explicitly freed.
> +///
> +/// Copying, cloning, or moving the allocator must not invalidate memory blocks returned from this
> +/// allocator. A copied, cloned or even new allocator of the same type must behave like the same
> +/// allocator, and any pointer to a memory buffer which is currently allocated may be passed to any
> +/// other method of the allocator.

If you provide no receiver methods, then I think we can remove this
requirement.

> +pub unsafe trait Allocator {
> +    /// Allocate memory based on `layout` and `flags`.
> +    ///
> +    /// On success, returns a buffer represented as `NonNull<[u8]>` that satisfies the size an

typo "an" -> "and"

> +    /// alignment requirements of layout, but may exceed the requested size.

Also if it may exceed the size, then I wouldn't call that "satisfies the
size [...] requirements".

> +    ///
> +    /// This function is equivalent to `realloc` when called with a NULL pointer and an `old_size`
> +    /// of `0`.

This is only true for the default implementation and could be
overridden, since it is not a requirement of implementing this trait to
keep it this way. I would remove this sentence.

> +    fn alloc(&self, layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {

Instead of using the `Flags` type from the alloc module, we should have
an associated `Flags` type in this trait.

Similarly, it might also be a good idea to let the implementer specify a
custom error type.

> +        // SAFETY: Passing a NULL pointer to `realloc` is valid by it's safety requirements and asks
> +        // for a new memory allocation.
> +        unsafe { self.realloc(ptr::null_mut(), 0, layout, flags) }
> +    }
> +
> +    /// Re-allocate an existing memory allocation to satisfy the requested `layout`. If the
> +    /// requested size is zero, `realloc` behaves equivalent to `free`.

This is not guaranteed by the implementation.

> +    ///
> +    /// If the requested size is larger than `old_size`, 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 `old_size`, `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 of a size of at least `old_size`.
> +    ///
> +    /// Additionally, `ptr` is allowed to be a NULL pointer; in this case a new memory allocation is
> +    /// created.
> +    unsafe fn realloc(
> +        &self,
> +        ptr: *mut u8,
> +        old_size: usize,

Why not request the old layout like the std Allocator's grow/shrink
functions do?

> +        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`
> +    /// instance.
> +    unsafe fn free(&self, ptr: *mut u8) {

`ptr` should be `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(ptr, 0, Layout::new::<()>(), Flags(0)) };

Why does the implementer have to guarantee this?

> +    }
> +}
> --
> 2.45.2
> 

More general questions:
- are there functions in the kernel to efficiently allocate zeroed
  memory? In that case, the Allocator trait should also have methods
  that do that (with a iterating default impl).
- I am not sure putting everything into the single realloc function is a
  good idea, I like the grow/shrink methods of the std allocator. Is
  there a reason aside from concentrating the impl to go for only a
  single realloc function?

---
Cheers,
Benno