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

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Message-ID: <0bb10df6-1e4d-4765-86c8-3549d2b778a2@huawei.com>
Date: Tue, 26 Nov 2024 22:28:04 +0800
From: Baokun Li <libaokun1@...wei.com>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>, <linux-ext4@...r.kernel.org>, Jan
 Kara <jack@...e.com>
CC: Ritesh Harjani <ritesh.list@...il.com>, <linux-kernel@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>, Disha Goel <disgoel@...ux.ibm.com>, Yang
 Erkun <yangerkun@...wei.com>
Subject: Re: [PATCH v2 1/2] quota: flush quota_release_work upon quota
 writeback

On 2024/11/21 20:38, 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>
Looks good, thanks for the patch!

Reviewed-by: Baokun Li <libaokun1@...wei.com>
> ---
>   fs/quota/dquot.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
> index 3dd8d6f27725..f9578918cfb2 100644
> --- a/fs/quota/dquot.c
> +++ b/fs/quota/dquot.c
> @@ -688,6 +688,8 @@ int dquot_writeback_dquots(struct super_block *sb, int type)
>   
>   	WARN_ON_ONCE(!rwsem_is_locked(&sb->s_umount));
>   
> +	flush_delayed_work(&quota_release_work);
> +
>   	for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
>   		if (type != -1 && cnt != type)
>   			continue;