Date:   Fri, 17 Dec 2021 08:59:51 -0800
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Chao Yu <chao@kernel.org>
Cc:     niuzhiguo84@gmail.com, Jing.Xia@unisoc.com,
        linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH Vx 1/1] f2fs: Avoid deadlock between writeback
 and checkpoint
Message-ID: <YbzCB4ZmeFsWs1rq@google.com>
References: <1636438608-27597-1-git-send-email-niuzhiguo84@gmail.com>
 <YZU0TFBH6k2Q6fJZ@google.com>
 <e28d4963-d816-b568-dec8-60a79a9fe88d@kernel.org>
 <e25053e9-f97e-6a2f-3bac-acfcd689fdcb@kernel.org>
 <00db09fe-122a-2c2c-1c6c-b6987db3d78b@kernel.org>
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On 12/17, Chao Yu wrote:
> On 2021/11/20 9:50, Chao Yu wrote:
> > On 2021/11/18 14:46, Chao Yu wrote:
> > > On 2021/11/18 0:56, Jaegeuk Kim wrote:
> > > > On 11/09, niuzhiguo84@gmail.com wrote:
> > > > > From: Zhiguo Niu <zhiguo.niu@unisoc.com>
> > > > > 
> > > > > There could be a scenario as following:
> > > > > The inodeA and inodeB are in b_io queue of writeback
> > > > > inodeA : f2fs's node inode
> > > > > inodeB : a dir inode with only one dirty pages, and the node page
> > > > > of inodeB cached into inodeA
> > > > > 
> > > > > writeback:
> > > > > 
> > > > > wb_workfn
> > > > > wb_writeback
> > > > > blk_start_plug
> > > > >            loop {
> > > > >            queue_io
> > > > >            progress=__writeback_inodes_wb
> > > > >                    __writeback_single_inode
> > > > >                            do_writepages
> > > > >                                    f2fs_write_data_pages
> > > > >                                    wbc->pages_skipped +=get_dirty_pages
> > > > >                            inode->i_state &= ~dirty
> > > > >                    wrote++
> > > > >                    requeue_inode
> > > > >            }
> > > > > blk_finish_plug
> > > > > 
> > > > > checkpoint:
> > > > > 
> > > > > f2fs_write_checkpoint
> > > > > f2fs_sync_dirty_inodes
> > > > > filemap_fdatawrite
> > > > > do_writepages
> > > > > f2fs_write_data_pages
> > > > >            f2fs_write_single_data_page
> > > > >                    f2fs_do_write_data_page
> > > > >                            set_page_writeback
> > > > >                            f2fs_outplace_write_data
> > > > >                                    f2fs_update_data_blkaddr
> > > > >                                            f2fs_wait_on_page_writeback
> > > > >                    inode_dec_dirty_pages
> > > > > 
> > > > > 1. Writeback thread flush inodeA, and push it's bio request in task's plug;
> > > > > 2. Checkpoint thread writes inodeB's dirty page, and then wait its node
> > > > >        page writeback cached into inodeA which is in writeback task's plug
> > > > > 3. Writeback thread flush inodeB and skip writing the dirty page as
> > > > >        wb_sync_req[DATA] > 0.
> > > > > 4. As none of the inodeB's page is marked as PAGECACHE_TAG_DIRTY, writeback
> > > > >        thread clear inodeB's dirty state.
> > > > > 5. Then inodeB is moved from b_io to b_dirty because of pages_skipped > 0
> > > > >        as checkpoint thread is stuck before dec dirty_pages.
> > > > > 
> > > > > This patch collect correct pages_skipped according to the tag state in
> > > > > page tree of inode
> > > > > 
> > > > > Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
> > > > > Signed-off-by: Jing Xia <jing.xia@unisoc.com>
> > > > > ---
> > > > >     fs/f2fs/data.c | 4 +++-
> > > > >     1 file changed, 3 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > > > > index f4fd6c246c9a..e98628e3868c 100644
> > > > > --- a/fs/f2fs/data.c
> > > > > +++ b/fs/f2fs/data.c
> > > > > @@ -3237,7 +3237,9 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
> > > > >     	return ret;
> > > > >     skip_write:
> > > > > -	wbc->pages_skipped += get_dirty_pages(inode);
> > > > > +	wbc->pages_skipped +=
> > > > > +		mapping_tagged(inode->i_mapping, PAGECACHE_TAG_DIRTY) ?
> > > > 
> > > > Is there any race condition to get 0, if there's any dirty page? IOWs, it
> > > 
> > > Quoted from Jing Xia's explanation:
> > > 
> > > [T:writeback]				[T:checkpoint]
> > 
> > My bad, [1] should be here:
> > 
> > bio contains NodeA was plugged in writeback threads
> > 
> > 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
> 
> Jaegeuk,
> 
> checkpoint() will stuck on waiting NodeA to writeback.
> 
> > > 					   - inode_dec_dirty_pages
> > 
> > > bio contains NodeA was plugged in writeback threads
> > 
> > [1]
> > 
> > Thanks,
> > 
> > > - 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
> 
> Meanwhile, kworker plugged NodeA, and then loop writebacking inode due to
> inode will be requeued all the time.

I'm thinking that this will be released after finishing node writes in a
different context. Then, this loop can happen in that time period which
is NOT forever. But, if we want to avoid this temporary loop, we may be
able to decrease the dirty count before f2fs_update_data_blkaddr().

> 
> Thanks,
> 
> > > 
> > > > seems the current condition is just requeuing the inode as dirty, but next
> > > > flushing time will remove it from dirty list. Is this giving too much overheads?
> > > 
> > > I prefer to let writeback thread call blk_flush_plug() after skipping
> > > writepages() due to wb_sync_req[DATA/NODE] check condition, thoughts?
> > > 
> > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > > index 9f754aaef558..b6e1ed73f8f5 100644
> > > --- a/fs/f2fs/data.c
> > > +++ b/fs/f2fs/data.c
> > > @@ -3087,6 +3087,8 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
> > >     			/* give a priority to WB_SYNC threads */
> > >     			if (atomic_read(&sbi->wb_sync_req[DATA]) &&
> > >     					wbc->sync_mode == WB_SYNC_NONE) {
> > > +				if (current->plug)
> > > +					blk_flush_plug(current->plug, false);
> > >     				done = 1;
> > >     				break;
> > >     			}
> > > diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> > > index 556fcd8457f3..dd9a817d8dab 100644
> > > --- a/fs/f2fs/node.c
> > > +++ b/fs/f2fs/node.c
> > > @@ -1946,6 +1946,8 @@ int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
> > >     			if (atomic_read(&sbi->wb_sync_req[NODE]) &&
> > >     					wbc->sync_mode == WB_SYNC_NONE) {
> > >     				done = 1;
> > > +				if (current->plug)
> > > +					blk_flush_plug(current->plug, false);
> > >     				break;
> > >     			}
> > > 
> > > 
> > > 
> > > Thanks,
> > > 
> > > > 
> > > > > +		get_dirty_pages(inode) : 0;
> > > > >     	trace_f2fs_writepages(mapping->host, wbc, DATA);
> > > > >     	return 0;
> > > > >     }
> > > > > -- 
> > > > > 2.28.0
> > > 
> > > 
> > > _______________________________________________
> > > Linux-f2fs-devel mailing list
> > > Linux-f2fs-devel@lists.sourceforge.net
> > > https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
> > > 
> > 
> > 
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