linux-ext4 - Re: [PATCH 7/9] gfs2: handle a NULL folio in gfs2_jhead_process

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Date:   Wed, 18 Jan 2023 16:00:33 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, linux-afs@...ts.infradead.org,
        linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        cluster-devel@...hat.com, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-nilfs@...r.kernel.org
Subject: Re: [PATCH 7/9] gfs2: handle a NULL folio in gfs2_jhead_process_page

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