Re: [PATCH] Btrfs: fix negative qgroup tracking from owner accounting (bug #61951)

[Jan Schmidt <list.btrfs@jan-o-sch.net>]

On Thu, October 24, 2013 at 16:49 (+0200), Wang Shilong wrote:
> Hello Jan,
> 
>> btrfs_dec_ref() queued a delayed ref for owner of a tree block. The qgroup
>> tracking is based on delayed refs. The owner of a tree block is set when a
>> tree block is allocated, it is never updated.
>>
>> When you allocate a tree block and then remove the subvolume that did the
>> allocation, the qgroup accounting for that removal is correct. However, the
>> removal was accounted again for each subvolume deletion that also referenced
>> the tree block, because accounting was erroneously based on the owner.
>>
>> Instead of queueing delayed refs for the non-existent owner, we now
>> queue delayed refs for the root being removed. This fixes the qgroup
>> accounting.
> 
> Thanks for tracking this, i apply your patch, and using the flowing patch,
> found the problem still exist, the test script like the following:

Reproduced. Gives more negative numbers due to accounting triggered by the
cleaner thread, that's the common part here. I still believe that the fix I sent
is correct, it's probably not complete. Looking into it.

Thanks,
-Jan

> #!/bin/sh
> 
> for i in $(seq 1000)
> do
> 	dd if=/dev/zero of=<mnt>/$i""aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa  bs=10K count=1
> done
> 
> btrfs sub snapshot <mnt> <mnt>/1
> for i in $(seq 100)
> do
> 	btrfs sub snapshot <mnt>/$i <mnt>/$(($i+1))
> done
> 
> for i in $(seq 101)
> do
> 	btrfs sub delete <mnt>/$i
> done
> 
> 
> Thanks,
> Wang