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

[Boqun Feng <boqun.feng@gmail.com>]

HI Onur,

On Mon, Oct 06, 2025 at 07:30:22PM +0300, 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.
> 

Having this information is good, however I feel it's better if you
explain/expand what exact the usage will be on the XArray, otherwise,
it'll be hard for people to dig in the history and find out why we add
this. Thanks!

Regards,
Boqun

> 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>> {
> +        build_assert!(
> +            T::FOREIGN_ALIGN >= 4,
> +            "pointers stored in XArray must be 4-byte aligned"
> +        );
> +
> +        let new = value.into_foreign();
> +        let mut id: u32 = 0;
> +
> +        // SAFETY:
> +        // - `self.xa.xa` is valid by the type invariant.
> +        // - `new` came from `T::into_foreign`.
> +        let ret =
> +            unsafe { bindings::__xa_alloc(self.xa.xa.get(), &mut id, new, limit, gfp.as_raw()) };
> +
> +        if ret < 0 {
> +            // SAFETY: `__xa_alloc` doesn't take ownership on error.
> +            let value = unsafe { T::from_foreign(new) };
> +            return Err(StoreError {
> +                value,
> +                error: Error::from_errno(ret),
> +            });
> +        }
> +
> +        Ok(id)
> +    }
>  }
> 
>  // SAFETY: `XArray<T>` has no shared mutable state so it is `Send` iff `T` is `Send`.
> --
> 2.51.0
>