On Thu, Jun 13, 2013 at 05:09:03PM -0500, Ben Myers wrote: > Hi Dave, > > On Thu, Jun 13, 2013 at 12:08:27PM +1000, Dave Chinner wrote: > > On Wed, Jun 12, 2013 at 08:04:41PM -0500, Ben Myers wrote: > > > On Wed, Jun 12, 2013 at 12:19:06PM +1000, Dave Chinner wrote: > > > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > > > > > Unfortunately, we cannot guarantee that items logged multiple times > > > > and replayed by log recovery do not take objects back in time. When > > > > theya re taken back in time, the go into an intermediate state which > > > > is corrupt, and hence verification that occurs on this intermediate > > > > state causes log recovery to abort with a corruption shutdown. > > > > > > > > Instead of causing a shutdown and unmountable filesystem, don't > > > > verify post-recovery items before they are written to disk. This is > > > > less than optimal, but there is no way to detect this issue for > > > > non-CRC filesystems If log recovery successfully completes, this > > > > will be undone and the object will be consistent by subsequent > > > > transactions that are replayed, so in most cases we don't need to > > > > take drastic action. > > > > > > > > For CRC enabled filesystems, leave the verifiers in place - we need > > > > to call them to recalculate the CRCs on the objects anyway. This > > > > recovery problem canbe solved for such filesystems - we have a LSN > > > > stamped in all metadata at writeback time that we can to determine > > > > whether the item should be replayed or not. This is a separate piece > > > > of work, so is not addressed by this patch. > > > > > > Is there a test case for this one? How are you reproducing this? > > > > The test case was Dave Jones running sysrq-b on a hung test machine. > > The machine would occasionally end up with a corrupt home directory. > > > > http://oss.sgi.com/pipermail/xfs/2013-May/026759.html > > > > Analysis from a metdadump provided by Dave: > > > > http://oss.sgi.com/pipermail/xfs/2013-June/026965.html > > > > And Cai also appeared to be hitting this after a crash on 3.10-rc4, > > as it's giving exactly the same "verifier failed during log recovery" > > stack trace: > > > > http://oss.sgi.com/pipermail/xfs/2013-June/026889.html > > Thanks. It appears that the verifiers have found corruption due to a > flaw in log recovery, and the fix you are proposing is to stop using > them. If we do that, we'll have no way of detecting the corruption and > will end up hanging users of older kernels out to dry. We've never detected it before, and it's causing regressions for multiple people. We *can't fix it* because we can't detect the situation sanely, and we are not leaving people with old kernels hanging out to dry. The opposite is true: we are fucking over current users by preventing log recovery on filesystems that will recovery perfectly OK and have almost always recovered just fine in the past. > I think your suggestion that non-debug systems could warn instead of > fail is a good one, but removing the verifier altogether is > inappropriate. Changing every single verifier in a non-trivial way is not something I'm about to do for a -rc6 kernel. Removing the verifiers from log recovery just reverts to the pre-3.8 situation, so is perfectly acceptable short term solution while we do the more invasive verify changes. > Can you make the metadump available? I need to understand this better > before I can sign off. Also: Any idea how far back this one goes? No, I can't make the metadump available to you - it was provided privately and not obfuscated and so you'd have to ask Dave for it. As to how long this problem has existed? It's a zero-day bug. Like I said, I've suspected for years that this can happen, and only now do we have proof of it... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
