Re: [PATCH -RFC 0/2] mm/ext4: avoid data corruption when extending DIO write race with buffered read

Jan Kara
2023-12-12T14:56:52Z

On Tue 12-12-23 04:36:45, Matthew Wilcox wrote:
> On Wed, Dec 06, 2023 at 08:37:57PM +0100, Jan Kara wrote:
> > Within the same page buffered reads and writes should be consistent because
> > they are synchronized by the page lock. However once reads and writes
> > involve multiple pages, there is no serialization so you can get contents
> > of some pages before write and some pages after being written. However this
> > doesn't seem to be your particular case here. I just wanted to point out
> > that in general even buffered reads vs writes are not fully consistent.
>
> Buffered reads don't take the page/folio lock. We only use the folio
> lock to avoid reading stale data from the page cache while we're
> fetching the data from storage. Once the uptodate flag is set on the
> folio, we never take the folio lock for reads.

Right, I've noticed once I've reread the code but thanks for correction
anyway. :)

Honza

--
Jan Kara <jack@suse.com>
SUSE Labs, CR