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SubjectRe: 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@mit.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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