Re: [PATCH 1/3] xfs: don't shutdown log recovery on validation errors

[Dave Chinner <david@fromorbit.com>]

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@redhat.com>
> > > > 
> > > > 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@fromorbit.com

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