linux-kernel - Re: [PATCH v2 1/2] quota: flush quota_release

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Message-ID: <20241126084744.fjnl3mmgme2mqhh2@quack3>
Date: Tue, 26 Nov 2024 09:47:44 +0100
From: Jan Kara <jack@...e.cz>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc: linux-ext4@...r.kernel.org, Jan Kara <jack@...e.com>,
	Ritesh Harjani <ritesh.list@...il.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Baokun Li <libaokun1@...wei.com>,
	Disha Goel <disgoel@...ux.ibm.com>
Subject: Re: [PATCH v2 1/2] quota: flush quota_release_work upon quota
 writeback

On Thu 21-11-24 18:08:54, Ojaswin Mujoo wrote:
> One of the paths quota writeback is called from is:
> 
> freeze_super()
>   sync_filesystem()
>     ext4_sync_fs()
>       dquot_writeback_dquots()
> 
> Since we currently don't always flush the quota_release_work queue in
> this path, we can end up with the following race:
> 
>  1. dquot are added to releasing_dquots list during regular operations.
>  2. FS Freeze starts, however, this does not flush the quota_release_work queue.
>  3. Freeze completes.
>  4. Kernel eventually tries to flush the workqueue while FS is frozen which
>     hits a WARN_ON since transaction gets started during frozen state:
> 
>   ext4_journal_check_start+0x28/0x110 [ext4] (unreliable)
>   __ext4_journal_start_sb+0x64/0x1c0 [ext4]
>   ext4_release_dquot+0x90/0x1d0 [ext4]
>   quota_release_workfn+0x43c/0x4d0
> 
> Which is the following line:
> 
>   WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);
> 
> Which ultimately results in generic/390 failing due to dmesg
> noise. This was detected on powerpc machine 15 cores.
> 
> To avoid this, make sure to flush the workqueue during
> dquot_writeback_dquots() so we dont have any pending workitems after
> freeze.
> 
> Reported-by: Disha Goel <disgoel@...ux.ibm.com>
> Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>

Thanks. Since this patch is independent, I've picked it up into my tree
(will push it to Linus for rc2).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR