On Mon, Mar 22, 2021 at 03:13:54PM -0700, Darrick J. Wong wrote:
> On Tue, Mar 23, 2021 at 08:30:16AM +1100, Dave Chinner wrote:
> > On Sat, Mar 20, 2021 at 09:40:07AM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <djwong@xxxxxxxxxx>
> > >
> > > While running some fuzz tests on inode metadata, I noticed that the
> > > filesystem health report (as provided by xfs_spaceman) failed to report
> > > the file corruption even when spaceman was run immediately after running
> > > xfs_scrub to detect the corruption. That isn't the intended behavior;
> > > one ought to be able to run scrub to detect errors in the ondisk
> > > metadata and be able to access to those reports for some time after the
> > > scrub.
> > >
> > > After running the same sequence through an instrumented kernel, I
> > > discovered the reason why -- scrub igets the file, scans it, marks it
> > > sick, and ireleases the inode. When the VFS lets go of the incore
> > > inode, it moves to RECLAIMABLE state. If spaceman igets the incore
> > > inode before it moves to RECLAIM state, iget reinitializes the VFS
> > > state, clears the sick and checked masks, and hands back the inode. At
> > > this point, the caller has the exact same incore inode, but with all the
> > > health state erased.
> > >
> > > In other words, we're erasing the incore inode's health state flags when
> > > we've decided NOT to sever the link between the incore inode and the
> > > ondisk inode. This is wrong, so we need to remove the lines that zero
> > > the fields from xfs_iget_cache_hit.
> > >
> > > As a precaution, we add the same lines into xfs_reclaim_inode just after
> > > we sever the link between incore and ondisk inode. Strictly speaking
> > > this isn't necessary because once an inode has gone through reclaim it
> > > must go through xfs_inode_alloc (which also zeroes the state) and
> > > xfs_iget is careful to check for mismatches between the inode it pulls
> > > out of the radix tree and the one it wants.
> > >
> > > Fixes: 6772c1f11206 ("xfs: track metadata health status")
> > > Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> > > ---
> > > fs/xfs/xfs_icache.c | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> > > index 595bda69b18d..5325fa28d099 100644
> > > --- a/fs/xfs/xfs_icache.c
> > > +++ b/fs/xfs/xfs_icache.c
> > > @@ -587,8 +587,6 @@ xfs_iget_cache_hit(
> > > ip->i_flags |= XFS_INEW;
> > > xfs_inode_clear_reclaim_tag(pag, ip->i_ino);
> > > inode->i_state = I_NEW;
> > > - ip->i_sick = 0;
> > > - ip->i_checked = 0;
> > >
> > > spin_unlock(&ip->i_flags_lock);
> > > spin_unlock(&pag->pag_ici_lock);
> > > @@ -1205,6 +1203,8 @@ xfs_reclaim_inode(
> > > spin_lock(&ip->i_flags_lock);
> > > ip->i_flags = XFS_IRECLAIM;
> > > ip->i_ino = 0;
> > > + ip->i_sick = 0;
> > > + ip->i_checked = 0;
> > > spin_unlock(&ip->i_flags_lock);
> > >
> > > xfs_iunlock(ip, XFS_ILOCK_EXCL);
> >
> > This is only going to keep the health information around on a
> > DONTCACHE inode for a few extra seconds. If the scrub takes longer
> > to run than it takes for the background inode reclaimer thread to
> > run again (every 5s by default), then the health information for
> > that inode is still trashed by this patch and the problem still
> > exists.
> >
> > I suspect that unhealthy inodes need to have IDONTCACHE cleared so
> > that they don't get reclaimed until there is memory pressure, hence
> > giving scrub/spaceman some time to set/report health issues.
>
> Yes, it seems reasonable to cancel DONTCACHE if you're marking an inode
> sick, and for iget to ignore DONTCACHE if the inode is in memory and is
> marked sick. This also sounds like a second patch. :)
Yup, sounds fine to me.
> > Perhaps we should not reclaim the unhealthy inodes until they've been
> > marked as "seen"....
>
> I'm hesitant to pin an inode in memory if it's unhealthy, because that
> seems like it could lead to further problems if a large number of inodes
> get marked sick and memory reclaim can't free enough RAM to enable a
> recovery action (like shutting down the fs and unmounting it).
This is a solvable problem because we can tell the difference
between background inode reclaim and memory pressure triggered inode
reclaim. firstly, memory pressure will release the last VFS
reference to the inode, so it's not seem by background reclaim until
memory reclaim has released it from the VFS. Secondly, we have
different entry points into inode reclaim from the shrinker vs
backgroun reclaim so we can have different behaviour for the two if
we need to.
I suspect that having the VFS LRUs hold on to sick inodes until
memory pressure occurs is more than sufficient to avoid most
"reclaimed before reported" events. IF we are in a "lots of
corruption" or online repair scenario, then the admin already knows
that bad stuff is going down and verbosely reporting all the sick
inodes isn't really necessary...
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
