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Date: Fri, 3 Dec 2021 14:44:00 +0800
From: Chao Yu <chao@...nel.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>
Cc: niuzhiguo84@...il.com, 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 2021/12/3 2:15, Jaegeuk Kim wrote:
> On 12/02, Chao Yu wrote:
>> On 2021/12/2 6:20, Jaegeuk Kim wrote:
>>> On 11/20, 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
>>>>> - 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
>>>
>>> So, my question was why this is the problem?
>>
>> kworker will loop writebacking this requeued inode.
>
> Does it make a problem?
The problem here is kworker will loop for ever.
Thanks,
>
>>
>> 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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