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

Danilo Krummrich
2024-08-04T12:30:01Z

On Sat, Aug 03, 2024 at 11:21:21PM -0700, Boqun Feng wrote:
> 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?

I will extend it to:

"valid to be passed to any other [`Allocator`] function" of the same type.

>
> 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
> >
>