linux-kernel - Re: [PATCH] f2fs: fix to avoid potential deadlock

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Date:   Thu, 3 Feb 2022 22:57:51 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, Zhiguo Niu <zhiguo.niu@...soc.com>,
        Jing Xia <jing.xia@...soc.com>
Subject: Re: [PATCH] f2fs: fix to avoid potential deadlock

On 2022/2/3 9:51, Jaegeuk Kim wrote:
> On 01/29, Chao Yu wrote:
>> On 2022/1/29 8:37, Jaegeuk Kim wrote:
>>> On 01/28, Chao Yu wrote:
>>>> On 2022/1/28 5:59, Jaegeuk Kim wrote:
>>>>> On 01/27, Chao Yu wrote:
>>>>>> Quoted from Jing Xia's report, there is a potential deadlock may happen
>>>>>> between kworker and checkpoint as below:
>>>>>>
>>>>>> [T:writeback]				[T:checkpoint]
>>>>>> - wb_writeback
>>>>>>     - blk_start_plug
>>>>>> bio contains NodeA was plugged in writeback threads
>>>>>
>>>>> I'm still trying to understand more precisely. So, how is it possible to
>>>>> have bio having node write in this current context?
>>>>
>>>> IMO, after above blk_start_plug(), it may plug some inode's node page in kworker
>>>> during writebacking node_inode's data page (which should be node page)?
>>>
>>> Wasn't that added into a different task->plug?
>>
>> I'm not sure I've got your concern correctly...
>>
>> Do you mean NodeA and other IOs from do_writepages() were plugged in
>> different local plug variables?
> 
> I think so.

I guess block plug helper says it doesn't allow to use nested plug, so there
is only one plug in kworker thread?

void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
{
	struct task_struct *tsk = current;

	/*
	 * If this is a nested plug, don't actually assign it.
	 */
	if (tsk->plug)
		return;
...
}

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>>> 					- do_writepages  -- sync write inodeB, inc wb_sync_req[DATA]
>>>>>> 					 - f2fs_write_data_pages
>>>>>> 					  - f2fs_write_single_data_page -- write last dirty page
>>>>>> 					   - f2fs_do_write_data_page
>>>>>> 					    - set_page_writeback  -- clear page dirty flag and
>>>>>> 					    PAGECACHE_TAG_DIRTY tag in radix tree
>>>>>> 					    - f2fs_outplace_write_data
>>>>>> 					     - f2fs_update_data_blkaddr
>>>>>> 					      - f2fs_wait_on_page_writeback -- wait NodeA to writeback here
>>>>>> 					   - inode_dec_dirty_pages
>>>>>>     - writeback_sb_inodes
>>>>>>      - writeback_single_inode
>>>>>>       - do_writepages
>>>>>>        - f2fs_write_data_pages -- skip writepages due to wb_sync_req[DATA]
>>>>>>         - wbc->pages_skipped += get_dirty_pages() -- PAGECACHE_TAG_DIRTY is not set but get_dirty_pages() returns one
>>>>>>      - requeue_inode -- requeue inode to wb->b_dirty queue due to non-zero.pages_skipped
>>>>>>     - blk_finish_plug
>>>>>>
>>>>>> Let's try to avoid deadlock condition by forcing unplugging previous bio via
>>>>>> blk_finish_plug(current->plug) once we'v skipped writeback in writepages()
>>>>>> due to valid sbi->wb_sync_req[DATA/NODE].
>>>>>>
>>>>>> Fixes: 687de7f1010c ("f2fs: avoid IO split due to mixed WB_SYNC_ALL and WB_SYNC_NONE")
>>>>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@...soc.com>
>>>>>> Signed-off-by: Jing Xia <jing.xia@...soc.com>
>>>>>> Signed-off-by: Chao Yu <chao@...nel.org>
>>>>>> ---
>>>>>>     fs/f2fs/data.c | 6 +++++-
>>>>>>     fs/f2fs/node.c | 6 +++++-
>>>>>>     2 files changed, 10 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>>> index 76d6fe7b0c8f..932a4c81acaf 100644
>>>>>> --- a/fs/f2fs/data.c
>>>>>> +++ b/fs/f2fs/data.c
>>>>>> @@ -3174,8 +3174,12 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
>>>>>>     	/* to avoid spliting IOs due to mixed WB_SYNC_ALL and WB_SYNC_NONE */
>>>>>>     	if (wbc->sync_mode == WB_SYNC_ALL)
>>>>>>     		atomic_inc(&sbi->wb_sync_req[DATA]);
>>>>>> -	else if (atomic_read(&sbi->wb_sync_req[DATA]))
>>>>>> +	else if (atomic_read(&sbi->wb_sync_req[DATA])) {
>>>>>> +		/* to avoid potential deadlock */
>>>>>> +		if (current->plug)
>>>>>> +			blk_finish_plug(current->plug);
>>>>>>     		goto skip_write;
>>>>>> +	}
>>>>>>     	if (__should_serialize_io(inode, wbc)) {
>>>>>>     		mutex_lock(&sbi->writepages);
>>>>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>>>>>> index 556fcd8457f3..69c6bcaf5aae 100644
>>>>>> --- a/fs/f2fs/node.c
>>>>>> +++ b/fs/f2fs/node.c
>>>>>> @@ -2106,8 +2106,12 @@ static int f2fs_write_node_pages(struct address_space *mapping,
>>>>>>     	if (wbc->sync_mode == WB_SYNC_ALL)
>>>>>>     		atomic_inc(&sbi->wb_sync_req[NODE]);
>>>>>> -	else if (atomic_read(&sbi->wb_sync_req[NODE]))
>>>>>> +	else if (atomic_read(&sbi->wb_sync_req[NODE])) {
>>>>>> +		/* to avoid potential deadlock */
>>>>>> +		if (current->plug)
>>>>>> +			blk_finish_plug(current->plug);
>>>>>>     		goto skip_write;
>>>>>> +	}
>>>>>>     	trace_f2fs_writepages(mapping->host, wbc, NODE);
>>>>>> -- 
>>>>>> 2.32.0