linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: get out of a repeat loop when getting a locked data page

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Date:   Mon, 27 Mar 2023 08:30:33 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, stable@...r.kernel.org,
        willy@...radead.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: get out of a repeat loop when getting a
 locked data page

On 03/26, Chao Yu wrote:
> On 2023/3/24 5:39, Jaegeuk Kim wrote:
> > https://bugzilla.kernel.org/show_bug.cgi?id=216050
> > 
> > Somehow we're getting a page which has a different mapping.
> > Let's avoid the infinite loop.
> > 
> > Cc: <stable@...r.kernel.org>
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> >   fs/f2fs/data.c | 8 ++------
> >   1 file changed, 2 insertions(+), 6 deletions(-)
> > 
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index bf51e6e4eb64..80702c93e885 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -1329,18 +1329,14 @@ struct page *f2fs_get_lock_data_page(struct inode *inode, pgoff_t index,
> >   {
> >   	struct address_space *mapping = inode->i_mapping;
> >   	struct page *page;
> > -repeat:
> > +
> >   	page = f2fs_get_read_data_page(inode, index, 0, for_write, NULL);
> >   	if (IS_ERR(page))
> >   		return page;
> >   	/* wait for read completion */
> >   	lock_page(page);
> > -	if (unlikely(page->mapping != mapping)) {
> 
> How about using such logic only for move_data_page() to limit affect for
> other paths?

Why move_data_page() only? If this happens, we'll fall into a loop in anywhere?

> 
> Jaegeuk, any thoughts about why mapping is mismatch in between page's one and
> inode->i_mapping?

> 
> After several times code review, I didn't get any clue about why f2fs always
> get the different mapping in a loop.

I couldn't find the path to happen this. So weird. Please check the history in the
bug.

> 
> Maybe we can loop MM guys to check whether below folio_file_page() may return
> page which has different mapping?

Matthew may have some idea on this?

> 
> struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
> 		int fgp_flags, gfp_t gfp)
> {
> 	struct folio *folio;
> 
> 	folio = __filemap_get_folio(mapping, index, fgp_flags, gfp);
> 	if (IS_ERR(folio))
> 		return NULL;
> 	return folio_file_page(folio, index);
> }
> 
> Thanks,
> 
> > -		f2fs_put_page(page, 1);
> > -		goto repeat;
> > -	}
> > -	if (unlikely(!PageUptodate(page))) {
> > +	if (unlikely(page->mapping != mapping || !PageUptodate(page))) {
> >   		f2fs_put_page(page, 1);
> >   		return ERR_PTR(-EIO);
> >   	}