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

[Danilo Krummrich <dakr@kernel.org>]

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
> > 
>