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Message-ID: <dd504acbdfb0b926e54318fd6ea8f9d43af73634.camel@kernel.org>
Subject: Re: [PATCH] NFS: Fix nfs_netfs_issue_read() xarray locking for
 writeback interrupt
From: Jeff Layton <jlayton@kernel.org>
To: David Wysochanski <dwysocha@redhat.com>, David Howells
 <dhowells@redhat.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>, Trond Myklebust
	 <trond.myklebust@hammerspace.com>, linux-nfs@vger.kernel.org, 
	linux-cachefs@redhat.com
Date: Mon, 29 Jan 2024 12:44:17 -0500
In-Reply-To: <CALF+zOm9QBkf3PbchuVzkgYW1aEJWFFQ5JF0Y-_6BbPJke8CHw@mail.gmail.com>
References: <20240129154750.1245317-1-dwysocha@redhat.com>
	 <1526807.1706548521@warthog.procyon.org.uk>
	 <CALF+zOm9QBkf3PbchuVzkgYW1aEJWFFQ5JF0Y-_6BbPJke8CHw@mail.gmail.com>
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On Mon, 2024-01-29 at 12:34 -0500, David Wysochanski wrote:
> On Mon, Jan 29, 2024 at 12:15=E2=80=AFPM David Howells <dhowells@redhat.c=
om> wrote:
> >=20
> > Dave Wysochanski <dwysocha@redhat.com> wrote:
> >=20
> > > -     xas_lock(&xas);
> > > +     xas_lock_irqsave(&xas, flags);
> > >       xas_for_each(&xas, page, last) {
> >=20
> > You probably want to use RCU, not xas_lock().  The pages are locked and=
 so
> > cannot be evicted from the xarray.
> >=20
>=20
> I tried RCU originally and ran into a problem because NFS can schedule
> (see comment on line 328 below)
>=20
> 326         xas_lock_irqsave(&xas, flags);
> 327         xas_for_each(&xas, page, last) {
> 328                 /* nfs_read_add_folio() may schedule() due to pNFS
> layout and other RPCs  */
> 329                 xas_pause(&xas);
> 330                 xas_unlock_irqrestore(&xas, flags);
> 331                 err =3D nfs_read_add_folio(&pgio, ctx, page_folio(pag=
e));
> 332                 if (err < 0) {
> 333                         netfs->error =3D err;
> 334                         goto out;
> 335                 }
> 336                 xas_lock_irqsave(&xas, flags);
> 337         }
> 338         xas_unlock_irqrestore(&xas, flags);
>=20

Looking at it more closely, I think you might want to just use
xa_for_each_start(). That will do the traversal using the rcu_read_lock
under the hood, and you should be able to block on every iteration.

--=20
Jeff Layton <jlayton@kernel.org>