linux-ext4 - Re: Apparent serious progressive ext4 data corruption bug in 3.6.3 (and other stable branches?)

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Date:	Sat, 27 Oct 2012 16:19:33 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Nix <nix@...eri.org.uk>
CC:	"Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org
Subject: Re: Apparent serious progressive ext4 data corruption bug in 3.6.3
 (and other stable branches?)

On 10/27/12 1:47 PM, Nix wrote:
> On 27 Oct 2012, Theodore Ts'o said:
> 
>> On Sat, Oct 27, 2012 at 01:45:25PM +0100, Nix wrote:
>>> Ah! it's turned on by journal_async_commit. OK, that alone argues
>>> against use of journal_async_commit, tested or not, and I'd not have
>>> turned it on if I'd noticed that.
>>>
>>> (So, the combinations I'll be trying for effect on this bug are:
>>>
>>>  journal_async_commit (as now)
>>>  journal_checksum
>>>  none
>>
>> Can you also check and see whether the presence or absence of
>> "nobarrier" makes a difference?
> 
> Done. (Also checked the effect of your patches posted earlier this week:
> no effect, I'm afraid, certainly not under the fails-even-on-3.6.1 test
> I was carrying out, umount -l'ing /var as the very last thing I did
> before /sbin/reboot -f.)
> 
> nobarrier makes a difference that I, at least, did not expect:
> 
> [no options]                    No corruption
> 
> nobarrier                       No corruption
> 
>           journal_checksum      Corruption
>                                 Corrupted transaction, journal aborted
>                                 
> nobarrier,journal_checksum      Corruption
>                                 Corrupted transaction, journal aborted
> 
>           journal_async_commit  Corruption
>                                 Corrupted transaction, journal aborted
> 
> nobarrier,journal_async_commit  Corruption
>                                 No corrupted transaction or aborted journal

That's what we needed.  Woulda been great a few days ago ;)

In my testing journal_checksum is broken, and my bisection seems to
implicate

commit 119c0d4460b001e44b41dcf73dc6ee794b98bd31
Author: Theodore Ts'o <tytso@....edu>
Date:   Mon Feb 6 20:12:03 2012 -0500

    ext4: fold ext4_claim_inode into ext4_new_inode
    
as the culprit.  I haven't had time to look into why, yet.

-Eric

> I didn't expect the last case at all, and it adequately explains why you
> are mostly seeing corrupted journal messages in your tests but I was
> not. It also explains why when I saw this for the first time I was able
> to mount the resulting corrupted filesystem read-write and corrupt it
> further before I noticed that anything was wrong.
> 
> It is also clear that journal_checksum and all that relies on it is
> worse than useless right now, as Eric reported while I was testing this.
> It should probably be marked CONFIG_BROKEN in future 3.[346].* stable
> kernels, if CONFIG_BROKEN existed anymore, which it doesn't.
> 
> It's a shame journal_async_commit depends on a broken feature: it might
> be notionally unsafe but on some of my systems (without nobarrier or
> flashy caching controllers) it was associated with a noticeable speedup
> of metadata-heavy workloads -- though that was way back in 2009...
> however, "safety first" definitely applies in this case.
> 

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