diff mbox series

[7/9] gfs2: handle a NULL folio in gfs2_jhead_process_page

Message ID 20230118094329.9553-8-hch@lst.de (mailing list archive)
State New
Headers show
Series [1/9] mm: don't look at xarray value entries in split_huge_pages_in_file | expand

Commit Message

Christoph Hellwig Jan. 18, 2023, 9:43 a.m. UTC
filemap_get_folio can return NULL, so exit early for that case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/gfs2/lops.c | 2 ++
 1 file changed, 2 insertions(+)

Comments

Matthew Wilcox Jan. 18, 2023, 4 p.m. UTC | #1
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;
>  
>  	folio_wait_locked(folio);
>  	if (folio_test_error(folio))
> -- 
> 2.39.0
>
Andreas Gruenbacher Jan. 18, 2023, 4:24 p.m. UTC | #2
[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
Christoph Hellwig Jan. 18, 2023, 4:42 p.m. UTC | #3
On Wed, Jan 18, 2023 at 05:24:32PM +0100, Andreas Gruenbacher wrote:
> 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.

Ok, I'll drop this patch.
diff mbox series

Patch

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))