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From:   Andreas Gruenbacher <agruenba@redhat.com>
Date:   Wed, 18 Jan 2023 17:24:32 +0100
Message-ID: <CAHc6FU7Exz2kr+x7jvK1mi5ENOVCOXruP7qKSdPsyhSwmfMhDA@mail.gmail.com>
Subject: Re: [Cluster-devel] [PATCH 7/9] gfs2: handle a NULL folio in gfs2_jhead_process_page
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Christoph Hellwig <hch@lst.de>, linux-xfs@vger.kernel.org,
        linux-nilfs@vger.kernel.org, Hugh Dickins <hughd@google.com>,
        cluster-devel@redhat.com, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-ext4@vger.kernel.org, linux-afs@lists.infradead.org,
        linux-btrfs@vger.kernel.org
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[Christoph's email ended up in my spam folder; I hope that was a
one-time-only occurrence.]

On Wed, Jan 18, 2023 at 5:00 PM Matthew Wilcox <willy@infradead.org> wrote:
> On Wed, Jan 18, 2023 at 10:43:27AM +0100, Christoph Hellwig wrote:
> > filemap_get_folio can return NULL, so exit early for that case.
>
> I'm not sure it can return NULL in this specific case.  As I understand
> this code, we're scanning the journal looking for the log head.  We've
> just submitted the bio to read this page.  I suppose memory pressure
> could theoretically push the page out, but if it does, we're doing the
> wrong thing by just returning here; we need to retry reading the page.
>
> Assuming we're not willing to do the work to add that case, I think I'd
> rather see the crash in folio_wait_locked() than get data corruption
> from failing to find the head of the log.
>
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/gfs2/lops.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/fs/gfs2/lops.c b/fs/gfs2/lops.c
> > index 1902413d5d123e..51d4b610127cdb 100644
> > --- a/fs/gfs2/lops.c
> > +++ b/fs/gfs2/lops.c
> > @@ -472,6 +472,8 @@ static void gfs2_jhead_process_page(struct gfs2_jdesc *jd, unsigned long index,
> >       struct folio *folio;
> >
> >       folio = filemap_get_folio(jd->jd_inode->i_mapping, index);
> > +     if (!folio)
> > +             return;

We're actually still holding a reference to the folio from the
find_or_create_page() in gfs2_find_jhead() here, so we know that
memory pressure can't push the page out and filemap_get_folio() won't
return NULL.

> >
> >       folio_wait_locked(folio);
> >       if (folio_test_error(folio))
> > --
> > 2.39.0
> >
>

Thanks,
Andreas