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

[Boqun Feng]

On Thu, Aug 01, 2024 at 02:02:00AM +0200, Danilo Krummrich wrote:
[...]
> +/// 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.
> +///

Are you saying you could kmalloc() a memory buffer and pass it to a
vfree()? Or am I missing something here?

Regards,
Boqun

> +/// 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>;
> +
> +    /// 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)) };
> +    }
> +}
> -- 
> 2.45.2
>