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

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Message-ID: <e5702c85-9e69-4d53-9518-faf037dedad7@linux.ibm.com>
Date: Fri, 29 Nov 2024 13:30:04 +0530
From: Disha Goel <disgoel@...ux.ibm.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, Baokun Li <libaokun1@...wei.com>
Subject: Re: [PATCH v2 1/2] quota: flush quota_release_work upon quota
 writeback

On 21/11/24 6:08 pm, 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 for the fix patch Ojaswin.
I have tested the patch on powerpc machine, and it fixes the generic/390 test failure.

Tested-by: Disha Goel <disgoel@...ux.ibm.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;