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

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Date:   Thu, 16 Jul 2020 09:17:05 +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 2020/7/16 9:10, Jaegeuk Kim wrote:
> On 07/16, Chao Yu wrote:
>> On 2020/7/16 3:10, Jaegeuk Kim wrote:
>>> On 07/14, Chao Yu wrote:
>>>> On 2020/7/14 1:59, Jaegeuk Kim wrote:
>>>>> On 07/10, Chao Yu wrote:
>>>>>> 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;
>>>>>
>>>>> Hmm, this doesn't work. The writeback was called before put_super?
>>>>
>>>> Oops, still be confused about this issue. :(
>>>
>>> Huam, I hit the problem with the patch.
>>> I need to return back and think in other way. :(
>>
>> Still quota data was left? what about dentry?
> 
> Huh, at this time, only node and meta pages were left as dirty.

I saw both meta and node's writepages() called from background will skip
flushing dirty pages if dirty page count didn't touch the threshold.

	/* collect a number of dirty node pages and write together */
	if (wbc->sync_mode != WB_SYNC_ALL &&
			get_pages(sbi, F2FS_DIRTY_NODES) <
					nr_pages_to_skip(sbi, NODE))
		goto skip_write;

	/* collect a number of dirty meta pages and write together */
	if (wbc->sync_mode != WB_SYNC_ALL &&
			get_pages(sbi, F2FS_DIRTY_META) <
					nr_pages_to_skip(sbi, META))
		goto skip_write;

At that time, would syncfs (type sync command via shell) help?

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>> I'll try the original patch one more time.
>>>>>
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> 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))
>>>>>>>>>>>>>
>>>>>>>>>>> .
>>>>>>>>>>>
>>>>>>>>> .
>>>>>>>>>
>>>>>>> .
>>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>