Re: [f2fs-dev] [PATCH] f2fs: fix to avoid potential deadlock

[Chao Yu <chao@kernel.org>]

On 2022/3/2 13:26, Jaegeuk Kim wrote:
> On 03/02, Chao Yu wrote:
>> ping,
>>
>> On 2022/2/25 11:02, Chao Yu wrote:
>>> On 2022/2/3 22:57, Chao Yu wrote:
>>>> On 2022/2/3 9:51, Jaegeuk Kim wrote:
>>>>> On 01/29, Chao Yu wrote:
>>>>>> On 2022/1/29 8:37, Jaegeuk Kim wrote:
>>>>>>> On 01/28, Chao Yu wrote:
>>>>>>>> On 2022/1/28 5:59, Jaegeuk Kim wrote:
>>>>>>>>> On 01/27, Chao Yu wrote:
>>>>>>>>>> Quoted from Jing Xia's report, there is a potential deadlock may happen
>>>>>>>>>> between kworker and checkpoint as below:
>>>>>>>>>>
>>>>>>>>>> [T:writeback]                [T:checkpoint]
>>>>>>>>>> - wb_writeback
>>>>>>>>>>      - blk_start_plug
>>>>>>>>>> bio contains NodeA was plugged in writeback threads
>>>>>>>>>
>>>>>>>>> I'm still trying to understand more precisely. So, how is it possible to
>>>>>>>>> have bio having node write in this current context?
>>>>>>>>
>>>>>>>> IMO, after above blk_start_plug(), it may plug some inode's node page in kworker
>>>>>>>> during writebacking node_inode's data page (which should be node page)?
>>>>>>>
>>>>>>> Wasn't that added into a different task->plug?
>>>>>>
>>>>>> I'm not sure I've got your concern correctly...
>>>>>>
>>>>>> Do you mean NodeA and other IOs from do_writepages() were plugged in
>>>>>> different local plug variables?
>>>>>
>>>>> I think so.
>>>>
>>>> I guess block plug helper says it doesn't allow to use nested plug, so there
>>>> is only one plug in kworker thread?
> 
> Is there only one kworker thread that flushes node and inode pages?

IIRC, =one kworker per block device?

Thanks,

> 
>>>>
>>>> void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
>>>> {
>>>>       struct task_struct *tsk = current;
>>>>
>>>>       /*
>>>>        * If this is a nested plug, don't actually assign it.
>>>>        */
>>>>       if (tsk->plug)
>>>>           return;
>>>> ...
>>>> }
>>>
>>> Any further comments?
>>>
>>> Thanks,
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> 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
>>>>>>>>>>      - 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
>>>>>>>>>>      - blk_finish_plug
>>>>>>>>>>
>>>>>>>>>> Let's try to avoid deadlock condition by forcing unplugging previous bio via
>>>>>>>>>> blk_finish_plug(current->plug) once we'v skipped writeback in writepages()
>>>>>>>>>> due to valid sbi->wb_sync_req[DATA/NODE].
>>>>>>>>>>
>>>>>>>>>> Fixes: 687de7f1010c ("f2fs: avoid IO split due to mixed WB_SYNC_ALL and WB_SYNC_NONE")
>>>>>>>>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
>>>>>>>>>> Signed-off-by: Jing Xia <jing.xia@unisoc.com>
>>>>>>>>>> Signed-off-by: Chao Yu <chao@kernel.org>
>>>>>>>>>> ---
>>>>>>>>>>      fs/f2fs/data.c | 6 +++++-
>>>>>>>>>>      fs/f2fs/node.c | 6 +++++-
>>>>>>>>>>      2 files changed, 10 insertions(+), 2 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>>>>>>> index 76d6fe7b0c8f..932a4c81acaf 100644
>>>>>>>>>> --- a/fs/f2fs/data.c
>>>>>>>>>> +++ b/fs/f2fs/data.c
>>>>>>>>>> @@ -3174,8 +3174,12 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
>>>>>>>>>>          /* to avoid spliting IOs due to mixed WB_SYNC_ALL and WB_SYNC_NONE */
>>>>>>>>>>          if (wbc->sync_mode == WB_SYNC_ALL)
>>>>>>>>>>              atomic_inc(&sbi->wb_sync_req[DATA]);
>>>>>>>>>> -    else if (atomic_read(&sbi->wb_sync_req[DATA]))
>>>>>>>>>> +    else if (atomic_read(&sbi->wb_sync_req[DATA])) {
>>>>>>>>>> +        /* to avoid potential deadlock */
>>>>>>>>>> +        if (current->plug)
>>>>>>>>>> +            blk_finish_plug(current->plug);
>>>>>>>>>>              goto skip_write;
>>>>>>>>>> +    }
>>>>>>>>>>          if (__should_serialize_io(inode, wbc)) {
>>>>>>>>>>              mutex_lock(&sbi->writepages);
>>>>>>>>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>>>>>>>>>> index 556fcd8457f3..69c6bcaf5aae 100644
>>>>>>>>>> --- a/fs/f2fs/node.c
>>>>>>>>>> +++ b/fs/f2fs/node.c
>>>>>>>>>> @@ -2106,8 +2106,12 @@ static int f2fs_write_node_pages(struct address_space *mapping,
>>>>>>>>>>          if (wbc->sync_mode == WB_SYNC_ALL)
>>>>>>>>>>              atomic_inc(&sbi->wb_sync_req[NODE]);
>>>>>>>>>> -    else if (atomic_read(&sbi->wb_sync_req[NODE]))
>>>>>>>>>> +    else if (atomic_read(&sbi->wb_sync_req[NODE])) {
>>>>>>>>>> +        /* to avoid potential deadlock */
>>>>>>>>>> +        if (current->plug)
>>>>>>>>>> +            blk_finish_plug(current->plug);
>>>>>>>>>>              goto skip_write;
>>>>>>>>>> +    }
>>>>>>>>>>          trace_f2fs_writepages(mapping->host, wbc, NODE);
>>>>>>>>>> -- 
>>>>>>>>>> 2.32.0
>>>>
>>>>
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>>>
>>>
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