linux-kernel - Re: [PATCH v3 05/12] rust: xarray: use `xas_load` instead of `xa

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Message-ID: <CAJ-ks9n0-Uy0n7CsZwPr37PSc_wo5qYG2YGVsGN50oPZN4MrSg@mail.gmail.com>
Date: Wed, 11 Feb 2026 13:19:23 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Boqun Feng <boqun@...nel.org>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Alice Ryhl <aliceryhl@...gle.com>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>, 
	Daniel Gomez <da.gomez@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 05/12] rust: xarray: use `xas_load` instead of
 `xa_load` in `Guard::load`

On Wed, Feb 11, 2026 at 10:00 AM Boqun Feng <boqun@...nel.org> wrote:
>
> On Wed, Feb 11, 2026 at 09:32:36AM -0500, Liam R. Howlett wrote:
> > * Alice Ryhl <aliceryhl@...gle.com> [260210 16:34]:
> > > On Tue, Feb 10, 2026 at 10:23 PM Tamir Duberstein <tamird@...nel.org> wrote:
> > > >
> > > > On Tue, Feb 10, 2026 at 12:59 PM Liam R. Howlett
> > > > <Liam.Howlett@...cle.com> wrote:
> > > > > Is this a temporary limitation?
> > > >
> > > > Maybe? I don't think RfL has good abstractions for RCU yet. For
> > > > example, exposing load directly on the xarray using xa_load would
> > > > require a way to guarantee that the returned pointer's target isn't
>
> Well, if we only return a pointer, we don't need to guarantee that,
> right? Because it's up to the user to provide that guarantee. So we
> could have XArray::load() (not Guard::load()) that just calls xa_load().
> Also see below.
>
> > > > being concurrently mutated (e.g. under the xarray lock). I'm not aware
> > > > of anyone asking for this, though.
> > >
> > > It's relatively easy to add an rcu-backed load using the RCU
> > > abstractions we have today. I already shared an RFC containing such a
> > > method for the maple tree, and it would not be much different for
> > > xarray.
> > > https://lore.kernel.org/all/20260116-rcu-box-v1-0-38ebfbcd53f0@google.com/
>
> I need to point out a difference between xas_load() and Alice's usage
> (also what Tamir mentioned above) there, what Alice needs (at least from
> her patchset) is the existence of the object is protected by RCU, i.e.
> if there is someone else dropping the object, a RCU read lock would
> still guarantee the access to the object is valid.
>
> However, the internal RCU usage of both xarray and maple tree is to
> protect the *internal* data structure if I'm not missing anything, i.e.
> an writer may change the array or the tree while a reader is reading,
> the internal structure itself is still consistent and valid. But the
> nothing guarantees the object you read is still valid. For example, you
> can have an xa_erase() racing with an xa_load():
>
>         <writer>                        <reader>
>         ptr = xa_erase(xa, idx);
>                                         ptr = xa_load(xa, idx);
>         reclaim(ptr);
>                                         use(ptr); // <- object may be gone
>
> the users of xarray needs to use other mechanism to guarantee the
> existence of the object.
>
> In Alice's case, she in fact used an RCU read side critical section with
> a larger scope to protect the object as well, which is definitely nice
> to have, but not only way of using maple/xarray.
>
> > >
> >
> > It would probably be worth having two loads then, one that does
> > rcu_read_lock()/unlock() and one for writer/advanced users like we have
> > on the C side of things.
> >
>
> Agreed. But we may need more ;-)
>
> Here IIUC that Andreas does is adding a `load()` for `Guard` of
> `XArray`, which is the load for a writer and most certainly you won't
> need to take an RCU read lock for that. The load of a reader can be
> added as I suggested above (similar as your "rcu_read_lock()/unlock()"
> suggestion above), but no object existence guarantee. We likely
> need a third API that can provide the object existence similar to what
> Alice had in maple tree.
>
> > Or at least name the load() function to indicate which is implemented
> > today?
> >
>
> It's a namespace thing ;-) , the function in this patch is
> kernel::xarray::Guard::load(), and as I suggest here
> kernel::xarray::XArray::load() should be the same as xa_load().

Just to clarify: `kernel::xarray::XArray::load()` does not currently
exist and no one has yet asked for it.

>
> Regards,
> Boqun
>
> > At least on the maple tree side, we have both interfaces and users for
> > both.  I just found the change to remove the rcu safety odd because I
> > assumed both are needed.
> >
> > Thanks,
> > Liam
>