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Date:   Thu, 3 Mar 2022 10:32:33 +0800
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Subject: Re: [f2fs-dev] [PATCH] f2fs: fix to avoid potential deadlock
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To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     Jing Xia <jing.xia@unisoc.com>,
        linux-f2fs-devel@lists.sourceforge.net,
        Zhiguo Niu <zhiguo.niu@unisoc.com>,
        linux-kernel@vger.kernel.org
References: <YfMVxzdhat01ca7m@google.com>
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 <86a175d3-c438-505b-1dbc-4ef6e8b5adcb@kernel.org>
 <5b5e20d1-877f-b321-b341-c0f233ee976c@kernel.org>
 <51826b5f-e480-994a-4a72-39ff4572bb3f@kernel.org>
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 <c0d3528b-e6b4-8557-4c2b-e26a972d8aaa@kernel.org>
 <Yh/JSlaIw49gV+15@google.com>
From:   Chao Yu <chao@kernel.org>
In-Reply-To: <Yh/JSlaIw49gV+15@google.com>
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On 2022/3/3 3:45, Jaegeuk Kim wrote:
> On 03/02, Chao Yu wrote:
>> 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?
> 
> If there's one kworker only, f2fs_write_node_pages() should have flushed its
> plug?

No, f2fs_write_node_pages() failed to attach local plug into current->plug due to
current has attached plug from wb_writeback(), and also, f2fs_write_node_pages()
will fail to flush current->plug due to its local plug doesn't match current->plug.

void blk_start_plug_nr_ios()
{
	if (tsk->plug)
		return;
...
}

void blk_finish_plug(struct blk_plug *plug)
{
	if (plug == current->plug) {
		__blk_flush_plug(plug, false);
		current->plug = NULL;
	}
}

Thanks,

> 
>>
>> 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
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Linux-f2fs-devel mailing list
>>>>>> Linux-f2fs-devel@lists.sourceforge.net
>>>>>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Linux-f2fs-devel mailing list
>>>>> Linux-f2fs-devel@lists.sourceforge.net
>>>>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel