Re: Enabling quota may not correctly rescan on 4.17

[Nikolay Borisov <nborisov@suse.com>]

On 26.06.2018 09:00, Misono Tomohiro wrote:
> Hello Nikolay,
> 
> I noticed that commit 5d23515be669 ("btrfs: Move qgroup rescan
> on quota enable to btrfs_quota_enable") in 4.17 sometimes causes
> to fail correctly rescanning quota when quota is enabled.
> 
> Simple reproducer:
> 
> $ mkfs.btrfs -f $DEV
> $ mount $DEV /mnt
> $ dd if=/dev/urandom of=/mnt/file bs=1000 count=1000
> $ btrfs quota enbale /mnt
> $ umount /mnt
> $ btrfs check $DEV
> ...
> checking quota groups
> Counts for qgroup id: 0/5 are different
> our:            referenced 1019904 referenced compressed 1019904
> disk:           referenced 16384 referenced compressed 16384
> diff:           referenced 1003520 referenced compressed 1003520
> our:            exclusive 1019904 exclusive compressed 1019904
> disk:           exclusive 16384 exclusive compressed 16384
> diff:           exclusive 1003520 exclusive compressed 1003520
> found 1413120 bytes used, error(s) found
> ...

I ran your script 100 times with progs 4.17 and 4.18-rc1 and didn't
observe this error. I didn't observe btrfs/114 also failing but I ran it
a lot less. Is there anything else i can do to make your small
reproducer more likely to trigger?

> 
> This can be also observed in btrfs/114. \v(Note that progs < 4.17
> returns error code 0 even if quota is not consistency and therefore
> test will incorrectly pass.)
> 
> My observation is that this commit changed to call initial quota rescan
> when quota is enabeld instead of first comit transaction after enabling
> quota, and therefore if there is something not commited at that time,
> their usage will not be accounted.
> 
> Actually this can be simply fixed by calling "btrfs rescan" again or
> calling "btrfs fi sync" before "btrfs quota enable".
> 
> I think the commit itself makes the code much easier to read, so it may
> be better to fix the problem in progs (i.e. calling sync before quota enable).
> 
> Do you have any thoughts?
> 
> Thanks,
> Tomohiro Misono
> 
> 
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