Date:   Wed, 1 Dec 2021 14:20:23 -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: <Yaf1J/GtTrJekmtn@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>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <e25053e9-f97e-6a2f-3bac-acfcd689fdcb@kernel.org>
Precedence: bulk

On 11/20, 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
> > 					   - 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

So, my question was why this is the problem?

> > 
> > > 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
> >