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Message-ID: <f0fa20e0-7c03-c454-d5a7-62457663412b@kernel.org>
Date: Thu, 2 Dec 2021 10:01:19 +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 2021/12/2 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.
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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