Re: kernel BUG at linux-4.2.0/fs/btrfs/extent-tree.c:1833 on rebalance

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Stéphane Lesimple wrote on 2015/09/17 10:02 +0200:
> Le 2015-09-17 08:42, Qu Wenruo a écrit :
>> Stéphane Lesimple wrote on 2015/09/17 08:11 +0200:
>>> Le 2015-09-17 05:03, Qu Wenruo a écrit :
>>>> Stéphane Lesimple wrote on 2015/09/16 22:41 +0200:
>>>>> Le 2015-09-16 22:18, Duncan a écrit :
>>>>>> Stéphane Lesimple posted on Wed, 16 Sep 2015 15:04:20 +0200 as
>>>>>> excerpted:
>>>>>>
>>>>>
>>>>> Well actually it's the (d) option ;)
>>>>> I activate the quota feature for only one reason : being able to track
>>>>> down how much space my snapshots are taking.
>>>>
>>>> Yeah, that's completely one of the ideal use case of btrfs qgroup.
>>>>
>>>> But I'm quite curious about the btrfsck error report on qgroup.
>>>>
>>>> If btrfsck report such error, it means either I'm too confident about
>>>> the recent qgroup accounting rework, or btrfsck has some bug which I
>>>> didn't take much consideration during the kernel rework.
>>>>
>>>> Would you please provide the full result of previous btrfsck with
>>>> qgroup error?
>>>
>>> Sure, I've saved the log somewhere just in case, here your are :
>>>
>>> [...]
>> Thanks for your log, pretty interesting result.
>>
>> BTW, did you enabled qgroup from old kernel earlier than 4.2-rc1?
>> If so, I would be much relaxed as they can be the problem of old kernels.
>
> The mkfs.btrfs was done under 3.19, but I'm almost sure I enabled quota
> under 4.2.0 precisely. My kern.log tends to confirm that (looking for
> 'qgroup scan completed').

Emmm, seems I need to pay more attention on this case now.
Any info about the workload for this btrfs fs?

>
>> If it's OK for you, would you please enable quota after reproducing
>> the bug and use for sometime and recheck it?
>
> Sure, I've just reproduced the bug twice as I wanted, and posted the
> info, so now I've cancelled the balance and I can reenable quota. Will
> do it under 4.3.0-rc1. I'll keep you posted if btrfsck complains about
> it in the following days.
>
> Regards,
>
Thanks for your patience and detailed report.

But I still have another question, did you do any snapshot deletion 
after quota enabled?
(I'll assume you did it, as there are a lot of backup snapshot, old ones 
should be already deleted)

That's one of the known bug and Mark is working on it actively.
If you delete non-empty snapshot a lot, then I'd better add a hot fix to 
mark qgroup inconsistent after snapshot delete, and trigger a rescan if 
possible.

Thanks,
Qu