Re: [PATCH 3/3] xfs: Fix stale data exposure when readahead races with hole punch

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Jul 12, 2019 at 02:00:04PM +0200, Jan Kara wrote:
> On Thu 11-07-19 08:49:17, Darrick J. Wong wrote:
> > On Thu, Jul 11, 2019 at 06:28:54PM +0300, Amir Goldstein wrote:
> > > > +{
> > > > +       struct xfs_inode *ip = XFS_I(file_inode(file));
> > > > +       int ret;
> > > > +
> > > > +       /* Readahead needs protection from hole punching and similar ops */
> > > > +       if (advice == POSIX_FADV_WILLNEED)
> > > > +               xfs_ilock(ip, XFS_IOLOCK_SHARED);
> > 
> > It's good to fix this race, but at the same time I wonder what's the
> > impact to processes writing to one part of a file waiting on IOLOCK_EXCL
> > while readahead holds IOLOCK_SHARED?
> > 
> > (bluh bluh range locks ftw bluh bluh)
> 
> Yeah, with range locks this would have less impact. Also note that we hold
> the lock only during page setup and IO submission. IO itself will already
> happen without IOLOCK, only under page lock. But that's enough to stop the
> race.

> > Do we need a lock for DONTNEED?  I think the answer is that you have to
> > lock the page to drop it and that will protect us from <myriad punch and
> > truncate spaghetti> ... ?
> 
> Yeah, DONTNEED is just page writeback + invalidate. So page lock is enough
> to protect from anything bad. Essentially we need IOLOCK only to protect
> the places that creates new pages in page cache.
> 
> > > > +       ret = generic_fadvise(file, start, end, advice);
> > > > +       if (advice == POSIX_FADV_WILLNEED)
> > > > +               xfs_iunlock(ip, XFS_IOLOCK_SHARED);
> > 
> > Maybe it'd be better to do:
> > 
> > 	int	lockflags = 0;
> > 
> > 	if (advice == POSIX_FADV_WILLNEED) {
> > 		lockflags = XFS_IOLOCK_SHARED;
> > 		xfs_ilock(ip, lockflags);
> > 	}
> > 
> > 	ret = generic_fadvise(file, start, end, advice);
> > 
> > 	if (lockflags)
> > 		xfs_iunlock(ip, lockflags);
> > 
> > Just in case we some day want more or different types of inode locks?
> 
> OK, will do. Just I'll get to testing this only after I return from
> vacation.

<nod>

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