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

[Chao Yu <chao@kernel.org>]

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?
>>
>> 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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