Re: [PATCH v2 2/4] rust: xarray: abstract `xa_alloc`

["Onur Özkan" <work@onurozkan.dev>]

On Wed, 8 Oct 2025 13:45:53 -0700
Tamir Duberstein <tamird@gmail.com> wrote:

> On Wed, Oct 8, 2025 at 12:50 PM Onur Özkan <work@onurozkan.dev> wrote:
> >
> > On Wed, 8 Oct 2025 09:59:12 -0700
> > Tamir Duberstein <tamird@gmail.com> wrote:
> >
> > > On Wed, Oct 8, 2025 at 6:05 AM Onur Özkan <work@onurozkan.dev>
> > > wrote:
> > > >
> > > > Implements `alloc` function to `XArray<T>` that wraps
> > > > `xa_alloc` safely, which will be used to generate the
> > > > auxiliary device IDs.
> > > >
> > > > 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 | 41
> > > > ++++++++++++++++++++++++++++++++++++++++- 1 file changed, 40
> > > > insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> > > > index 90e27cd5197e..0711ccf99fb4 100644
> > > > --- a/rust/kernel/xarray.rs
> > > > +++ b/rust/kernel/xarray.rs
> > > > @@ -10,7 +10,7 @@
> > > >      ffi::c_void,
> > > >      types::{ForeignOwnable, NotThreadSafe, Opaque},
> > > >  };
> > > > -use core::{iter, marker::PhantomData, pin::Pin, ptr::NonNull};
> > > > +use core::{iter, marker::PhantomData, ops::Range, pin::Pin,
> > > > ptr::NonNull}; use pin_init::{pin_data, pin_init, pinned_drop,
> > > > PinInit};
> > > >
> > > >  /// An array which efficiently maps sparse integer indices to
> > > > owned objects. @@ -268,6 +268,45 @@ pub fn store(
> > > >              Ok(unsafe { T::try_from_foreign(old) })
> > > >          }
> > > >      }
> > > > +
> > > > +    /// Allocates an empty slot within the given `limit` and
> > > > stores `value` there.
> > > > +    ///
> > > > +    /// May drop the lock if needed to allocate memory, and
> > > > then reacquire it afterwards.
> > > > +    ///
> > > > +    /// On success, returns the allocated index.
> > >
> > > Returning the index is not a very good abstraction. Would the
> > > reservation API meet your needs?
> > >
> > > https://lore.kernel.org/all/20250713-xarray-insert-reserve-v2-3-b939645808a2@gmail.com/
> > >
> > > If yes, I would appreciate your tags there.
> >
> > It should be "allocated key", I misdocumented it. I don't have a
> > use-case for this implementation, I am just trying to help on the
> > nova task list:
> >     https://docs.kernel.org/gpu/nova/core/todo.html#xarray-bindings-xarr
> 
> I think implementing things without understanding the use-case is a
> good way to build the wrong thing.
> 

I was thinking I would get some review notes from people who actually
need this if something wasn't right. Maybe Alexandre can clarify what
the expected outcome was, since he created the task.

Onur

> > The task mentions "generate the auxiliary device IDs", which should
> > be the returned key, right?
> 
> I dunno.
> 
> > There is also this reference [1] that shows that the returned key
> > will be useful.
> >
> > [1]: https://lore.kernel.org/all/aOTyVzpJNDOaxxs6@google.com/
> 
> Sure, it's useful - the reservation API also exposes it. But it is not
> a proper abstraction.
> 
> Cheers.
> 
> Tamir