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

[Chao Yu <chao@kernel.org>]

On 2021/11/20 9:50, Chao Yu wrote:
> 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

Jaegeuk,

checkpoint() will stuck on waiting NodeA to writeback.

>> 					   - 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

Meanwhile, kworker plugged NodeA, and then loop writebacking inode due to
inode will be requeued all the time.

Thanks,

>>
>>> 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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>>
> 
> 
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