Re: [PATCH 1/3] rust: xarray: abstract `xa_alloc`

["Onur Özkan" <work@onurozkan.dev>]

On Wed, 8 Oct 2025 13:01:34 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> On Wed, Oct 08, 2025 at 12:18:24PM +0200, Benno Lossin wrote:
> > On Tue Oct 7, 2025 at 12:58 PM CEST, Alice Ryhl wrote:
> > > On Mon, Oct 06, 2025 at 09:31:43PM +0200, Benno Lossin wrote:
> > >> On Mon Oct 6, 2025 at 6:30 PM CEST, Onur Özkan wrote:
> > >> > Implements `alloc` function to `XArray<T>` that wraps
> > >> > `xa_alloc` safely.
> > >> >
> > >> > Resolves a task from the nova/core task list under the "XArray
> > >> > bindings [XARR]" section in
> > >> > "Documentation/gpu/nova/core/todo.rst" file.
> > >> >
> > >> > Signed-off-by: Onur Özkan <work@onurozkan.dev>
> > >> > ---
> > >> >  rust/kernel/xarray.rs | 39
> > >> > +++++++++++++++++++++++++++++++++++++++ 1 file changed, 39
> > >> > insertions(+)
> > >> >
> > >> > diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> > >> > index a49d6db28845..1b882cd2f58b 100644
> > >> > --- a/rust/kernel/xarray.rs
> > >> > +++ b/rust/kernel/xarray.rs
> > >> > @@ -266,6 +266,45 @@ pub fn store(
> > >> >              Ok(unsafe { T::try_from_foreign(old) })
> > >> >          }
> > >> >      }
> > >> > +
> > >> > +    /// Allocates an empty slot within the given limit range
> > >> > and stores `value` there.
> > >> > +    ///
> > >> > +    /// May drop the lock if needed to allocate memory, and
> > >> > then reacquire it afterwards.
> > >> > +    ///
> > >> > +    /// On success, returns the allocated id.
> > >> > +    ///
> > >> > +    /// On failure, returns the element which was attempted
> > >> > to be stored.
> > >> > +    pub fn alloc(
> > >> > +        &mut self,
> > >> > +        limit: bindings::xa_limit,
> > >> > +        value: T,
> > >> > +        gfp: alloc::Flags,
> > >> > +    ) -> Result<u32, StoreError<T>> {
> > >> 
> > >> I think it would be a good idea to make the id a newtype wrapper
> > >> around u32. Maybe not even allow users to manually construct it
> > >> or even inspect it if possible.
> > >
> > > What? People need to know what the assigned index is.
> > 
> > The documentation says "allocated id", so I assumed that it was some
> > internal, implementation-dependent thing, not an index. In that
> > case we should change the docs instead.
> 
> An xarray is a map from integer to pointer. The allocated id the key
> in this map. The alloc method picks the smallest unused key in a given
> range.
> 
> Alice

Perhaps we should document it as "allocated key" or "allocated id (key)"
?

-Onur