Re: [PATCH] common/quota: fix to wait for all inodes been evicted in _check_quota_usage()

[Chao Yu <chao@kernel.org>]

On 2025/12/5 05:59, Darrick J. Wong wrote:
> On Thu, Dec 04, 2025 at 03:31:47AM +0800, Chao Yu wrote:
>> generic/233  3s ... - output mismatch (see /share/git/fstests/results//generic/233.out.bad)
>>      --- tests/generic/233.out   2025-01-12 21:57:40.259440359 +0800
>>      +++ /share/git/fstests/results//generic/233.out.bad 2025-12-04 03:02:26.000000000 +0800
>>      @@ -4,4 +4,12 @@
>>
>>       seed = S
>>       Comparing user usage
>>      +4c4
>>      +< #1000     --   31476   32000   32000            994  1000  1000
>>      +---
>>      +> #1000     --   31476   32000   32000            944  1000  1000
>>      ...
>>      (Run 'diff -u /share/git/fstests/tests/generic/233.out /share/git/fstests/results//generic/233.out.bad'  to see the entire diff)
>> Ran: generic/233
>> Failures: generic/233
>> Failed 1 of 1 tests
>>
>> Sometimes, generic/233 will fail due to it founds inode count is mismatched
>> in between in-memory one and on-disk one.
>>
>> The reason is cgroup v2 implementation requires to increase inode reference,
>> and then attach it to thread related cgroup writeback structure, once it
>> needs to switch once write owner changes of target inode, a kernel thread
>> will process it and then release inode reference via evict_inode().
>>
>> So, sync & drop_cache may not guarantee all inodes being evicted, as cgroup
>> has one more refernece on inodes during the time.
>>
>> If inode has not been evicted, dquot inode reference will not be release, it
>> will lead to inode quota mismatch.
>>
>> Let's add a delay to wait for cgroup switching completion before quota check.
>>
>> Cc: Jaegeuk Kim <jaegeuk@kernel.org>
>> Signed-off-by: Chao Yu <chao@kernel.org>
>> ---
>>   common/quota | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/common/quota b/common/quota
>> index a51386b1..de7e84e8 100644
>> --- a/common/quota
>> +++ b/common/quota
>> @@ -333,6 +333,9 @@ _check_quota_usage()
>>   	# XXX: really need an ioctl instead of this big hammer
>>   	echo 3 > /proc/sys/vm/drop_caches
>>   
>> +	# wait for inode_switch_wbs_wor_fn() to release inodes
>> +	sleep 3
> 
> Odd -- I've never seen this problem crop up on ext4 or XFS.  What's
> different about f2fs such that writeback is still in progress?
> 
> Oh, XFS cycles the mount and ext4 does ... quotaoff???

f2fs does quotaoff like ext4 as well, but the difference ext4_quota_off() will call
sync_filesystem() before turning off quota, f2fs doesn't, not sure this is the key
to wait/speedup inode_switch_wbs_wor_fn() or not.

 > > Perhaps the sleep 3 thing should only be done for f2fs?

Yes, I think so, let me revise the patch.

> Or do you see it on other filesystems as well?

I tested ext4, it doesn't fail.

Thanks,

> 
> --D
> 
>> +
>>   	VFS_QUOTA=0
>>   	case $FSTYP in
>>   	ext2|ext3|ext4|f2fs|gfs2|bcachefs)
>> -- 
>> 2.49.0
>>
>>