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Date: Fri, 17 Dec 2021 22:44:50 +0800
From: Chao Yu <chao@...nel.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>, niuzhiguo84@...il.com
Cc: Jing.Xia@...soc.com, linux-kernel@...r.kernel.org,
linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH Vx 1/1] f2fs: Avoid deadlock between writeback
and checkpoint
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@...il.com wrote:
>>>> From: Zhiguo Niu <zhiguo.niu@...soc.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@...soc.com>
>>>> Signed-off-by: Jing Xia <jing.xia@...soc.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
>>
>>
>> _______________________________________________
>> Linux-f2fs-devel mailing list
>> Linux-f2fs-devel@...ts.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>>
>
>
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