linux-kernel - Re: [PATCH 1/3] rust: xarray: abstract `xa

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Message-ID: <aOZgrhsNrKpxM_PW@google.com>
Date: Wed, 8 Oct 2025 13:01:34 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <lossin@...nel.org>
Cc: "Onur Özkan" <work@...rozkan.dev>, rust-for-linux@...r.kernel.org, ojeda@...nel.org, 
	alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, 
	bjorn3_gh@...tonmail.com, tmgross@...ch.edu, dakr@...nel.org, 
	linux-kernel@...r.kernel.org, acourbot@...dia.com, airlied@...il.com, 
	simona@...ll.ch, maarten.lankhorst@...ux.intel.com, mripard@...nel.org, 
	tzimmermann@...e.de, corbet@....net, lyude@...hat.com, 
	linux-doc@...r.kernel.org
Subject: Re: [PATCH 1/3] rust: xarray: abstract `xa_alloc`

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@...rozkan.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