linux-kernel - Re: [PATCH v2 2/4] rust: xarray: abstract `xa

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Message-ID: <CAJ-ks9nMziN2LU=T=XGhV8xau6UfGXOMZ49+2Lrt8KGbL7Qngg@mail.gmail.com>
Date: Wed, 8 Oct 2025 13:45:53 -0700
From: Tamir Duberstein <tamird@...il.com>
To: Onur Özkan <work@...rozkan.dev>
Cc: rust-for-linux@...r.kernel.org, ojeda@...nel.org, alex.gaynor@...il.com, 
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, 
	lossin@...nel.org, aliceryhl@...gle.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, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/4] rust: xarray: abstract `xa_alloc`

On Wed, Oct 8, 2025 at 12:50 PM Onur Özkan <work@...rozkan.dev> wrote:
>
> On Wed, 8 Oct 2025 09:59:12 -0700
> Tamir Duberstein <tamird@...il.com> wrote:
>
> > On Wed, Oct 8, 2025 at 6:05 AM Onur Özkan <work@...rozkan.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@...rozkan.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.

> 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