Re: [PATCH 2/3] Btrfs: rework qgroup accounting

From: Josef Bacik <jbacik@fb.com>
To: Wang Shilong <wangshilong1991@gmail.com>
Cc: <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH 2/3] Btrfs: rework qgroup accounting
Date: Tue, 7 Jan 2014 11:43:16 -0500	[thread overview]
Message-ID: <52CC2EA4.50206@fb.com> (raw)
In-Reply-To: <A4C427B3-2FE2-4032-A085-77117A678B0F@gmail.com>

On 12/21/2013 03:56 AM, Wang Shilong wrote:
> Hello Josef,
>
> Though i know there are still problems related to qgroup(for example removing snapshot
> will beak qgroup accounting).I did a simple test about your patch..
>
>
> # btrfs quota enable /mnt
> # dd if=/dev/zero of=/mnt/data bs=4k count=102400 oflag=direct
> # btrfs sub snapshot /mnt/ /mnt/snap1
> # btrfs sub snapshot /mnt /mnt/snap2
> # btrfs sub delete /mnt/snap1 /mnt/snap2
> # sync
> # rm -rf /mnt/data
> # sync
> # dmesg
> # btrfs qgroup show /mnt
>
>
> Firstly, qgroup accounting is wrong, this is maybe expected because efficient  fs tree removal.
> However, from dmesg, i get the  WARNING:
>
> WARNING: CPU: 1 PID: 2650 at fs/btrfs/qgroup.c:1486 btrfs_delayed_qgroup_accounting
>
> I did not take a deep look at codes, but i think you will be interested in taking a look at this. ^_^

Ok I finally sat down to look at this and it is because subvol deletion 
screws up quota accounting no matter what.  I just added this WARN_ON() 
to catch actual mistakes, it just so happens it gets tripped when the 
snapshot deletion stuff happens too.  I'm going to put this on the "I'll 
deal with it later" list since it is an existing issue with or without 
my patch.  Thanks,

Josef