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Message-ID: <51826b5f-e480-994a-4a72-39ff4572bb3f@kernel.org>
Date: Wed, 2 Mar 2022 11:32:27 +0800
From: Chao Yu <chao@...nel.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>
Cc: Jing Xia <jing.xia@...soc.com>,
linux-f2fs-devel@...ts.sourceforge.net,
Zhiguo Niu <zhiguo.niu@...soc.com>,
linux-kernel@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix to avoid potential deadlock
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@...soc.com>
>>>>>>>> Signed-off-by: Jing Xia <jing.xia@...soc.com>
>>>>>>>> Signed-off-by: Chao Yu <chao@...nel.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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>> Linux-f2fs-devel@...ts.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>
>
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