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

[Chao Yu <chao@kernel.org>]

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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