Re: [PATCH v4 01/28] rust: alloc: add `Allocator` trait

Danilo Krummrich
2024-08-06T18:30:45Z

On Tue, Aug 06, 2024 at 04:03:49PM +0000, Benno Lossin wrote:
> On 05.08.24 17:19, 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@kernel.org>
> > ---
> > rust/kernel/alloc.rs | 79 ++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 79 insertions(+)
> >
> > diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> > index 1966bd407017..8a71a589469d 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 +87,81 @@ 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.
>
> This will prevent us from implementing arena-type allocators [^1]. Do we
> want/need those?

I'm not aware of any code in the kernel that does exactly this, but kmem_cache
is rather close to that.

> I have heard that some people use them in embedded systems, but I can't
> say for sure. But this is a rather big design decision, so we should
> discuss it now.
>
> [^1]: For those who don't know what I mean by that here is a quick
> sketch (without handling flags and optimizations):
>
> pub struct ArenaAlloc<const SIZE: usize> {
> memory: Opaque<[u8; SIZE]>,
> head: Cell<usize>,
> }
>
> impl<const SIZE: usize> ArenaAlloc<SIZE> {
> pub fn new() -> Self {
> Self {
> memory: Opaque::uninit(),
> head: 0,
> }
> }
> }
>
> impl<const SIZE: usize> Allocator for ArenaAlloc<SIZE> {
> fn alloc(&self, layout: Layout, _flags: Flags) -> Result<NonNull<u8>, AllocError> {
> let head = self.head.get();
> if head + layout.size() >= SIZE {
> return Err(AllocError);
> }
> let ptr = self.memory.get();
> let ptr = ptr.cast::<u8>();
> let ptr = unsafe { ptr.add(head) };
> self.head.set(head + layout.size());
> unsafe { NonNull::new_unchecked(ptr) }
> }
>
> unsafe fn realloc(
> &self,
> ptr: Option<NonNull<u8>>,
> old_layout: Layout, // Note that we also need `old_layout`!
> layout: Layout,
> flags: Flags
> ) -> Result<NonNull<u8>, AllocError> {
> let new = self.alloc(layout, flags)?;
> let Some(ptr) = ptr else { return Ok(new); };
> unsafe { core::ptr::copy_nonoverlapping(ptr.as_ptr(), new.as_ptr(), old_layout.size()) };
> self.free(ptr);
> Ok(new)
> }
>
> fn free(&self, ptr: NonNull<u8>) { /* noop */ }
> }
>
> > +///
> > +/// 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.
>
> Ah I see, so since `#[derive(SmartPointer)]` needs `Box` to only consist
> of one non ZST field... I skimmed the RFC discussion and it seems like a
> problem that *might* be solved in the future, but probably not in the
> (very) near future. I guess this is just a bullet that we have to bite.
> We can always have an `ArenaBox` that can deal with that (although
> without `DispatchFromDyn`).
> We should revisit this when `#[derive(SmartPointer)]` becomes advanced
> enough.

Agreed.

>
> > +///
> > +/// # 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 of the same type. 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
> > + ///
> > + /// `Some(ptr)` must point to an existing and valid memory allocation created by this allocator
>
> This is the wrong way around, `ptr: Option<NonNull<u8>>`, so
> `Some(ptr): Option<Option<NonNull<u8>>>`. Instead I would write
> "If `ptr = Some(p)`, then `p` must point to...".

Yes, makes sense.

>
> > + /// instance. 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.
> > + ///
> > + 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.
>
> Additionally, you need "The memory allocation at `ptr` must never again
> be read from or written to.".

I'm fine adding it, but I wonder if technically this is really required? The
condition whether the pointer is ever accessed again in any way is not relevant
in terms of being a precondition for `free` not causing UB, right?

>
> ---
> Cheers,
> Benno
>
> > + 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
> >
>