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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 15:22:23 -0500
Cc: Dave Jones <davej@xxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <51BB74F0.7040406@xxxxxxxxxxx>
References: <20130613010441.GX20932@xxxxxxx> <20130613020827.GG29338@dastard> <20130613220903.GA20932@xxxxxxx> <20130614001306.GM29338@dastard> <20130614160940.GA32736@xxxxxxx> <51BB41AD.4050303@xxxxxxxxxxx> <20130614190850.GB20932@xxxxxxx> <51BB6C7C.6050300@xxxxxxxxxxx> <20130614194453.GC20932@xxxxxxx> <51BB74F0.7040406@xxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
Hey Eric,

On Fri, Jun 14, 2013 at 02:54:24PM -0500, Eric Sandeen wrote:
> On 6/14/13 2:44 PM, Ben Myers wrote:
> > Hey Eric,
> > 
> > On Fri, Jun 14, 2013 at 02:18:20PM -0500, Eric Sandeen wrote:
> >> On 6/14/13 2:08 PM, Ben Myers wrote:
> >>> On Fri, Jun 14, 2013 at 11:15:41AM -0500, Eric Sandeen wrote:
> >>>> 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 detected a an inconsistent *interim* state during replay, which is
> >> always made correct by log replay completion.  But it *stopped* that log
> >> replay completion.  And caused log replay to fail.  And mount to fail.
> >> This is *new* behavior, and bad.
> >>
> >> As I understand it.
> >>
> >>> 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 is not spin.
> >>
> >>> 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.
> >>
> >> Right.
> >>
> >> And it's been around for years.  The verifier now detects that
> >> interim state, and makes things *worse* than they would be had log
> >> replay been allowed to continue.
> >>
> >> Fixing the interim state may be nontrivial; allowing log replay
> >> to continue to a consistent state as it always has *is* trivial,
> >> it's what's done in Dave's small patch.
> >>
> >>>> 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?  ;)
> >>
> >> We seem to be talking past each other.
> >>
> >> The corrupted interim state has been around for years.  Up until
> >> now, log replay completion left things in perfect state.
> >>
> >> The verifier now *breaks replay* at that interim point.
> >> Were it allowed to continue, everything would be fine.
> >>
> >> As things stand, it is not fine, and this is a recent change
> >> which Dave is trying to correct.
> >>
> >> Leaving it in place will cause filesystems which were replaying
> >> logs just fine until recently to now fail with no good way out.
> > 
> > That is consistent with my understanding of the problem...
> > 
> > Unfortunately log replay is broken.  The verifier has detected this and 
> > stopped
> > replay.  Ideally the solution would be to fix log replay, but that is going 
> > to
> > take some time.  So, in the near term we're just going to disable the 
> > verifier
> > to allow replay to complete.
> 
> Right, that's what we're hoping - for 3.10 right?

Yep.  But it should also go back through -stable.

> Maybe the talking-past-each-other was only that part.  I thought you didn't
> want to disable it for now.

I didn't want to disable it.  That was yesterday.  Mark set me straight on how
easy this should be to hit.

> > I'm suggesting that this disabling be done conditionally on 
> > CONFIG_XFS_DEBUG so
> > that developers still have a chance at hitting the log replay problem, 
> 
> so that real-world users will still feel the pain ...?
> 
> Or did you say that backwards (really only *disabling* it under debug?)
> 
> Ok, confirmed on IRC you mean to disable it if *NOT* debug, enable it
> under debug.

Right.

> > and a
> > comment should be added explaining that we've disabled the verifier due to a
> > specific bug as a temporary workaround and we'll re-enable the verifier once
> > it's fixed.  I'll update the patch and repost.
> 
> Maybe if the verifiers were *on* under debug that'd make sense.

Sorry for the confusion.

> I think putting it under the config is overkill, since anyone who wants
> to fix it is surely capable of re-enabling it in the code.  But if that
> avoids an impasse, I don't much care.

Great.  Apparently I'm being pedantic today.  I'd like it to be clear that we
intend for this to be a temporary workaround....

> > Are you guys arguing that the log replay bug should not be fixed?
> 
> Speaking for myself, I'm not arguing that, not at all.
> (not that I know how to fix it, either)

...until log replay is fixed.  Hell.  I'll just pull it in as-is.

Reviewed-by: Ben Myers <bpm@xxxxxxx>

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