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

[Jaegeuk Kim]

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?

> > > 
> > > 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@xxxxxxxxxx>
> > > > > > > > > Signed-off-by: Jing Xia <jing.xia@xxxxxxxxxx>
> > > > > > > > > Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
> > > > > > > > > ---
> > > > > > > > >     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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