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

To: Ben Myers <bpm@xxxxxxx>
Subject: Re: [PATCH 1/3] xfs: don't shutdown log recovery on validation errors
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Fri, 14 Jun 2013 14:54:24 -0500
Cc: Dave Jones <davej@xxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130614194453.GC20932@xxxxxxx>
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> <20130614190850.GB20932@xxxxxxx> <51BB6C7C.6050300@xxxxxxxxxxx> <20130614194453.GC20932@xxxxxxx>
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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?

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

> 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.

> 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.

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.

> 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)

-Eric

> -Ben
> 

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