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

[Chao Yu <chao@kernel.org>]

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]
					- 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
- 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