linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: don't skip writeback of quota data

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Date:   Fri, 10 Jul 2020 15:31:10 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>, <kernel-team@...roid.com>
Subject: Re: [f2fs-dev] [PATCH] f2fs: don't skip writeback of quota data

On 2020/7/10 11:50, Jaegeuk Kim wrote:
> On 07/10, Chao Yu wrote:
>> On 2020/7/10 11:26, Jaegeuk Kim wrote:
>>> On 07/10, Chao Yu wrote:
>>>> On 2020/7/10 3:05, Jaegeuk Kim wrote:
>>>>> On 07/09, Chao Yu wrote:
>>>>>> On 2020/7/9 13:30, Jaegeuk Kim wrote:
>>>>>>> It doesn't need to bypass flushing quota data in background.
>>>>>>
>>>>>> The condition is used to flush quota data in batch to avoid random
>>>>>> small-sized udpate, did you hit any problem here?
>>>>>
>>>>> I suspect this causes fault injection test being stuck by waiting for inode
>>>>> writeback completion. With this patch, it has been running w/o any issue so far.
>>>>> I keep an eye on this.
>>>>
>>>> Hmmm.. so that this patch may not fix the root cause, and it may hiding the
>>>> issue deeper.
>>>>
>>>> How about just keeping this patch in our private branch to let fault injection
>>>> test not be stuck? until we find the root cause in upstream codes.
>>>
>>> Well, I don't think this hides something. When the issue happens, I saw inodes
>>> being stuck due to writeback while only quota has some dirty data. At that time,
>>> there was no dirty data page from other inodes.
>>
>> Okay,
>>
>>>
>>> More specifically, I suspect __writeback_inodes_sb_nr() gives WB_SYNC_NONE and
>>> waits for wb_wait_for_completion().
>>
>> Did you record any callstack after the issue happened?
> 
> I found this.
> 
> [213389.297642]  __schedule+0x2dd/0x780^M
> [213389.299224]  schedule+0x55/0xc0^M
> [213389.300745]  wb_wait_for_completion+0x56/0x90^M
> [213389.302469]  ? wait_woken+0x80/0x80^M
> [213389.303997]  __writeback_inodes_sb_nr+0xa8/0xd0^M
> [213389.305760]  writeback_inodes_sb+0x4b/0x60^M
> [213389.307439]  sync_filesystem+0x2e/0xa0^M
> [213389.308999]  generic_shutdown_super+0x27/0x110^M
> [213389.310738]  kill_block_super+0x27/0x50^M
> [213389.312327]  kill_f2fs_super+0x76/0xe0 [f2fs]^M
> [213389.314014]  deactivate_locked_super+0x3b/0x80^M
> [213389.315692]  deactivate_super+0x3e/0x50^M
> [213389.317226]  cleanup_mnt+0x109/0x160^M
> [213389.318718]  __cleanup_mnt+0x12/0x20^M
> [213389.320177]  task_work_run+0x70/0xb0^M
> [213389.321609]  exit_to_usermode_loop+0x131/0x160^M
> [213389.323306]  do_syscall_64+0x170/0x1b0^M
> [213389.324762]  entry_SYSCALL_64_after_hwframe+0x44/0xa9^M
> [213389.326477] RIP: 0033:0x7fc4b5e6a35b^M

Does this only happen during umount? If so, will below change help?

	if ((S_ISDIR(inode->i_mode) || IS_NOQUOTA(inode)) &&
+			!is_sbi_flag_set(sbi, SBI_IS_CLOSE) &&
			wbc->sync_mode == WB_SYNC_NONE &&
			get_dirty_pages(inode) < nr_pages_to_skip(sbi, DATA) &&
			f2fs_available_free_memory(sbi, DIRTY_DENTS))
		goto skip_write;

> 
>>
>> Still I'm confused that why directory's data written could be skipped, but
>> quota's data couldn't, what's the difference?
> 
> I suspect different blocking timing from cp_error between quota and dentry.
> e.g., we block dir operations right after cp_error, while quota can make

No guarantee that there is no dirty dentry being created after
cp_error, right?

e.g.

Thread A				Thread B
- f2fs_create
- bypass f2fs_cp_error
					- set cp_error
- create dirty dentry

BTW, do you know what __writeback_inodes_sb_nr is waiting for?

> dirty pages in more fine granularity.
> 
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
>>>>>>> ---
>>>>>>>  fs/f2fs/data.c | 2 +-
>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>>>> index 44645f4f914b6..72e8b50e588c1 100644
>>>>>>> --- a/fs/f2fs/data.c
>>>>>>> +++ b/fs/f2fs/data.c
>>>>>>> @@ -3148,7 +3148,7 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
>>>>>>>  	if (unlikely(is_sbi_flag_set(sbi, SBI_POR_DOING)))
>>>>>>>  		goto skip_write;
>>>>>>>  
>>>>>>> -	if ((S_ISDIR(inode->i_mode) || IS_NOQUOTA(inode)) &&
>>>>>>> +	if (S_ISDIR(inode->i_mode) &&
>>>>>>>  			wbc->sync_mode == WB_SYNC_NONE &&
>>>>>>>  			get_dirty_pages(inode) < nr_pages_to_skip(sbi, DATA) &&
>>>>>>>  			f2fs_available_free_memory(sbi, DIRTY_DENTS))
>>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>