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

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: [PATCH 1/3] xfs: don't shutdown log recovery on validation errors
From: Ben Myers <bpm@xxxxxxx>
Date: Fri, 14 Jun 2013 14:08:50 -0500
Cc: Dave Jones <davej@xxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <51BB41AD.4050303@xxxxxxxxxxx>
References: <1371003548-4026-1-git-send-email-david@xxxxxxxxxxxxx> <1371003548-4026-2-git-send-email-david@xxxxxxxxxxxxx> <20130613010441.GX20932@xxxxxxx> <20130613020827.GG29338@dastard> <20130613220903.GA20932@xxxxxxx> <20130614001306.GM29338@dastard> <20130614160940.GA32736@xxxxxxx> <51BB41AD.4050303@xxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
Hey Eric,

On Fri, Jun 14, 2013 at 11:15:41AM -0500, Eric Sandeen wrote:
> On 6/14/13 11:09 AM, Ben Myers wrote:
> > On Fri, Jun 14, 2013 at 10:13:06AM +1000, Dave Chinner wrote:
> >> On Thu, Jun 13, 2013 at 05:09:03PM -0500, Ben Myers wrote:
> >>> 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.
> > 
> > Dave (Jones), could you make the metadump available to me?  I'd like to
> > understand this a little bit better.  I'm a bit uncomfortable with the
> > proposition that we should corrupt silently in this case...
> 
> Ben, isn't it the case that the corruption would only happen if
> log replay failed for some reason (as has always been the case,
> verifier or not), but with the verifier in place, it kills replay
> even w/o other problems due to a logical problem with the
> (recently added) verifiers?

It seems like the verifier prevented corruption from hitting disk during
log replay.  It is enforcing a partial replay up to the point where the
corruption occurred.  Now you should be able to zero the log and the
filesystem is not corrupted.

> IOW - this seems like an actual functional regression due to the
> addition of the verifier, and dchinner's patch gets us back
> to the almost-always-fine state we were in prior to the change.

Oh, the spin doctor is *in*!

This isn't a logical problem with the verifier, it's a logical problem
with log replay.  We need to find a way for recovery to know whether a
given transaction should be replayed.  Fixing that is nontrivial.

> As we're at -rc6, it seems quite reasonable to me as a quick
> fix to just short-circuit it for now.

If we're talking about a short term fix, that's fine.  This should be
conditional on CONFIG_XFS_DEBUG and marked as such.

Long term, removing the verifiers is the wrong thing to do here.  We
need to fix the recovery bug and then remove this temporary workaround.  

> If you have time to analyze dave's metadump that's cool, but
> this seems like something that really needs to be addressed
> before 3.10 gets out the door.

If this really is a day one bug then it's been out the door almost
twenty years.  And you want to hurry now?  ;)

> Whenever you're ready, you can also add:
> 
> Reviewed-by: Eric Sandeen <sandeen@xxxxxxxxxx>

Sure.

> (And dchinner, should this cc: stable for 3.9?)

Looks like verifiers were added in 3.7.  We should Cc stable.

Thanks,
        Ben

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