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

Ben Myers
2013-06-14T20:22:27Z

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@sgi.com>