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

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Date:   Sat, 29 Jan 2022 09:48:25 +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 2022/1/29 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?

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