Re: [f2fs-dev] [PATCH Vx 1/1] f2fs: Avoid deadlock between writeback and checkpoint

[Chao Yu <chao@kernel.org>]

On 2021/11/18 14:46, Chao Yu wrote:
> On 2021/11/18 0:56, Jaegeuk Kim wrote:
>> On 11/09, niuzhiguo84@gmail.com wrote:
>>> From: Zhiguo Niu <zhiguo.niu@unisoc.com>
>>>
>>> There could be a scenario as following:
>>> The inodeA and inodeB are in b_io queue of writeback
>>> inodeA : f2fs's node inode
>>> inodeB : a dir inode with only one dirty pages, and the node page
>>> of inodeB cached into inodeA
>>>
>>> writeback:
>>>
>>> wb_workfn
>>> wb_writeback
>>> blk_start_plug
>>>           loop {
>>>           queue_io
>>>           progress=__writeback_inodes_wb
>>>                   __writeback_single_inode
>>>                           do_writepages
>>>                                   f2fs_write_data_pages
>>>                                   wbc->pages_skipped +=get_dirty_pages
>>>                           inode->i_state &= ~dirty
>>>                   wrote++
>>>                   requeue_inode
>>>           }
>>> blk_finish_plug
>>>
>>> checkpoint:
>>>
>>> f2fs_write_checkpoint
>>> f2fs_sync_dirty_inodes
>>> filemap_fdatawrite
>>> do_writepages
>>> f2fs_write_data_pages
>>>           f2fs_write_single_data_page
>>>                   f2fs_do_write_data_page
>>>                           set_page_writeback
>>>                           f2fs_outplace_write_data
>>>                                   f2fs_update_data_blkaddr
>>>                                           f2fs_wait_on_page_writeback
>>>                   inode_dec_dirty_pages
>>>
>>> 1. Writeback thread flush inodeA, and push it's bio request in task's plug;
>>> 2. Checkpoint thread writes inodeB's dirty page, and then wait its node
>>>       page writeback cached into inodeA which is in writeback task's plug
>>> 3. Writeback thread flush inodeB and skip writing the dirty page as
>>>       wb_sync_req[DATA] > 0.
>>> 4. As none of the inodeB's page is marked as PAGECACHE_TAG_DIRTY, writeback
>>>       thread clear inodeB's dirty state.
>>> 5. Then inodeB is moved from b_io to b_dirty because of pages_skipped > 0
>>>       as checkpoint thread is stuck before dec dirty_pages.
>>>
>>> This patch collect correct pages_skipped according to the tag state in
>>> page tree of inode
>>>
>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
>>> Signed-off-by: Jing Xia <jing.xia@unisoc.com>
>>> ---
>>>    fs/f2fs/data.c | 4 +++-
>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>> index f4fd6c246c9a..e98628e3868c 100644
>>> --- a/fs/f2fs/data.c
>>> +++ b/fs/f2fs/data.c
>>> @@ -3237,7 +3237,9 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
>>>    	return ret;
>>>    
>>>    skip_write:
>>> -	wbc->pages_skipped += get_dirty_pages(inode);
>>> +	wbc->pages_skipped +=
>>> +		mapping_tagged(inode->i_mapping, PAGECACHE_TAG_DIRTY) ?
>>
>> Is there any race condition to get 0, if there's any dirty page? IOWs, it
> 
> Quoted from Jing Xia's explanation:
> 
> [T:writeback]				[T:checkpoint]

My bad, [1] should be here:

bio contains NodeA was plugged in writeback threads

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

> bio contains NodeA was plugged in writeback threads

[1]

Thanks,

> - 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
> 
>> seems the current condition is just requeuing the inode as dirty, but next
>> flushing time will remove it from dirty list. Is this giving too much overheads?
> 
> I prefer to let writeback thread call blk_flush_plug() after skipping
> writepages() due to wb_sync_req[DATA/NODE] check condition, thoughts?
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 9f754aaef558..b6e1ed73f8f5 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -3087,6 +3087,8 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
>    			/* give a priority to WB_SYNC threads */
>    			if (atomic_read(&sbi->wb_sync_req[DATA]) &&
>    					wbc->sync_mode == WB_SYNC_NONE) {
> +				if (current->plug)
> +					blk_flush_plug(current->plug, false);
>    				done = 1;
>    				break;
>    			}
> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> index 556fcd8457f3..dd9a817d8dab 100644
> --- a/fs/f2fs/node.c
> +++ b/fs/f2fs/node.c
> @@ -1946,6 +1946,8 @@ int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
>    			if (atomic_read(&sbi->wb_sync_req[NODE]) &&
>    					wbc->sync_mode == WB_SYNC_NONE) {
>    				done = 1;
> +				if (current->plug)
> +					blk_flush_plug(current->plug, false);
>    				break;
>    			}
> 
> 
> 
> Thanks,
> 
>>
>>> +		get_dirty_pages(inode) : 0;
>>>    	trace_f2fs_writepages(mapping->host, wbc, DATA);
>>>    	return 0;
>>>    }
>>> -- 
>>> 2.28.0
> 
> 
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