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

[Chao Yu <chao@kernel.org>]

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)?

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@unisoc.com>
>> Signed-off-by: Jing Xia <jing.xia@unisoc.com>
>> Signed-off-by: Chao Yu <chao@kernel.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


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