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References: <20240129154750.1245317-1-dwysocha@redhat.com> <1526807.1706548521@warthog.procyon.org.uk>
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From: David Wysochanski <dwysocha@redhat.com>
Date: Mon, 29 Jan 2024 12:34:47 -0500
Message-ID: <CALF+zOm9QBkf3PbchuVzkgYW1aEJWFFQ5JF0Y-_6BbPJke8CHw@mail.gmail.com>
Subject: Re: [PATCH] NFS: Fix nfs_netfs_issue_read() xarray locking for
 writeback interrupt
To: David Howells <dhowells@redhat.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>, 
	Trond Myklebust <trond.myklebust@hammerspace.com>, Jeff Layton <jlayton@kernel.org>, 
	linux-nfs@vger.kernel.org, linux-cachefs@redhat.com
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On Mon, Jan 29, 2024 at 12:15=E2=80=AFPM David Howells <dhowells@redhat.com=
> wrote:
>
> Dave Wysochanski <dwysocha@redhat.com> wrote:
>
> > -     xas_lock(&xas);
> > +     xas_lock_irqsave(&xas, flags);
> >       xas_for_each(&xas, page, last) {
>
> You probably want to use RCU, not xas_lock().  The pages are locked and s=
o
> cannot be evicted from the xarray.
>

I tried RCU originally and ran into a problem because NFS can schedule
(see comment on line 328 below)

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(page)=
);
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);