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

[Chao Yu]

On 2023/3/27 23:30, Jaegeuk Kim wrote:
> 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@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > > ---
> > >    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?

Actually, we only suffer dead loop from foreground GC path, right? I suspect the
bug was triggered in this path only.

It looks there are a lot of cases in where we repeat triggering read once
two mappings are mismatched, e.g.
- __get_meta_page
- __get_node_page
- f2fs_write_begin
- f2fs_quota_read

Thanks,

> > 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);
> > >    	}