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

[Danilo Krummrich <dakr@kernel.org>]

On Thu, Aug 01, 2024 at 10:19:41AM +0200, Alice Ryhl wrote:
> On Thu, Aug 1, 2024 at 2:07 AM Danilo Krummrich <dakr@kernel.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@kernel.org>
> 
> It could be nice to mention that the design without `self` parameters
> is necessary for compatibility with #[derive(SmartPointer)].

Agreed, will do.

> 
> >  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.

Yes, it's intentional and yes it really misses a note in the documentation.

The idea is to allow a change of alignment as long as the new alignment is
smaller than the old alignment.

In terms of safety, it is the callers responsibility to ensure constant
alignment throughout re-allocations (if required).

> 
> > +    /// 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`.

Yes, I think that's fine though. Hopefully no one attempts to use the memory
anymore once `free` is being called.

> 
> Alice
>