Re: [PATCH v4] btrfs: qgroup: Fix qgroup accounting when creating snapshot

[Josef Bacik <jbacik@fb.com>]

On 04/24/2016 08:56 PM, Qu Wenruo wrote:
>
>
> Josef Bacik wrote on 2016/04/22 14:23 -0400:
>> On 04/22/2016 02:21 PM, Mark Fasheh wrote:
>>> On Fri, Apr 22, 2016 at 02:12:11PM -0400, Josef Bacik wrote:
>>>> On 04/15/2016 05:08 AM, Qu Wenruo wrote:
>>>>> +    /*
>>>>> +     * Force parent root to be updated, as we recorded it before so
>>>>> its
>>>>> +     * last_trans == cur_transid.
>>>>> +     * Or it won't be committed again onto disk after later
>>>>> +     * insert_dir_item()
>>>>> +     */
>>>>> +    if (!ret)
>>>>> +        record_root_in_trans(trans, parent, 1);
>>>>> +    return ret;
>>>>> +}
>>>>
>>>> NACK, holy shit we aren't adding a special transaction commit only
>>>> for qgroup snapshots.  Figure out a different way.  Thanks,
>>>
>>> Yeah I saw that. To be fair, we run a whole lot of the transaction stuff
>>> multiple times (at least from my reading) so I'm really unclear on
>>> what the
>>> performance impact is.
>>>
>>> Do you have any suggestion though? We've been banging our heads
>>> against this
>>> for a while now and as slow as this patch might be, it actually works
>>> where
>>> nothing else has so far.
>>
>> I'm less concerned about committing another transaction and more
>> concerned about the fact that it is an special variant of the
>> transaction commit.  If this goes wrong, or at some point in the future
>> we fail to update it along with btrfs_transaction_commit we suddenly are
>> corrupting metadata.  If we have to commit a transaction then call
>> btrfs_commit_transaction(), don't open code a stripped down version,
>> here be dragons.  Thanks,
>>
>> Josef
>>
>>
>>
> Yes, I also don't like the dirty hack.
>
> Although the problem is, we have no other good choice.
>
> If we can call commit_transaction() that's the best case, but the
> problem is, in create_pending_snapshots(), we are already inside
> commit_transaction().
>
> Or commit_transaction() can be called inside commit_transaction()?
>

No, figure out a different way.  IIRC I dealt with this with the 
no_quota flag for inc_ref/dec_ref since the copy root stuff does strange 
things with the reference counts, but all this code is gone now.  I 
looked around to see if I could figure out how the refs are ending up 
this way but it doesn't make sense to me and there isn't enough 
information in your changelog for me to be able to figure it out. 
You've created this mess, clean it up without making it messier.  Thanks,

Josef