linux-kernel - Re: [PATCH 2/3] writeback: update dirtied_when for synced inode to prevent livelock

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Date:	Thu, 5 May 2011 20:18:32 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/3] writeback: update dirtied_when for synced inode to
 prevent livelock

On Thu, May 05, 2011 at 05:10:50AM +0800, Jan Kara wrote:
> On Mon 02-05-11 11:17:52, Wu Fengguang wrote:
> > With the more aggressive "keep writeback as long as we wrote something"
> > logic in wb_writeback(), the "use LONG_MAX .nr_to_write" trick in commit
> > b9543dac5bbc ("writeback: avoid livelocking WB_SYNC_ALL writeback") is
> > no longer enough to stop sync livelock.
> > 
> > The fix is to explicitly update .dirtied_when on synced inodes, so that
> > they are no longer considered for writeback in the next round.
> Looks good. But shouldn't you also update i_dirtied_when in the case where
> !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY) but inode->i_state & I_DIRTY?
> Mostly for consistency since the chances of livelock because of this are
> really close to zero. 

Yes the I_DIRTY_SYNC may well be redirtied by the FS. It's very unlikely
to cause live locks, so I'm more than glad to kill the optional code :) 

Thanks,
Fengguang

> > CC: Jan Kara <jack@...e.cz>
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> >  fs/fs-writeback.c |    9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > ext3/ext4 are working fine now, however tests show that XFS may still
> > livelock inside the XFS routines:
> > 
> > [ 3581.181253] sync            D ffff8800b6ca15d8  4560  4403   4392 0x00000000
> > [ 3581.181734]  ffff88006f775bc8 0000000000000046 ffff8800b6ca12b8 00000001b6ca1938
> > [ 3581.182411]  ffff88006f774000 00000000001d2e40 00000000001d2e40 ffff8800b6ca1280
> > [ 3581.183088]  00000000001d2e40 ffff88006f775fd8 00000340af111ef2 00000000001d2e40
> > [ 3581.183765] Call Trace:
> > [ 3581.184008]  [<ffffffff8109be73>] ? lock_release_holdtime+0xa3/0xab
> > [ 3581.184392]  [<ffffffff8108cc0d>] ? prepare_to_wait+0x6c/0x79
> > [ 3581.184756]  [<ffffffff8108cc0d>] ? prepare_to_wait+0x6c/0x79
> > [ 3581.185120]  [<ffffffff812ed520>] xfs_ioend_wait+0x87/0x9f
> > [ 3581.185474]  [<ffffffff8108c97a>] ? wake_up_bit+0x2a/0x2a
> > [ 3581.185827]  [<ffffffff812f742a>] xfs_sync_inode_data+0x92/0x9d
> > [ 3581.186198]  [<ffffffff812f76e2>] xfs_inode_ag_walk+0x1a5/0x287
> > [ 3581.186569]  [<ffffffff812f779b>] ? xfs_inode_ag_walk+0x25e/0x287
> > [ 3581.186946]  [<ffffffff812f7398>] ? xfs_sync_worker+0x69/0x69
> > [ 3581.187311]  [<ffffffff812e2354>] ? xfs_perag_get+0x68/0xd0
> > [ 3581.187669]  [<ffffffff81092175>] ? local_clock+0x41/0x5a
> > [ 3581.188020]  [<ffffffff8109be73>] ? lock_release_holdtime+0xa3/0xab
> > [ 3581.188403]  [<ffffffff812e22ec>] ? xfs_check_sizes+0x160/0x160
> > [ 3581.188773]  [<ffffffff812e2354>] ? xfs_perag_get+0x68/0xd0
> > [ 3581.189130]  [<ffffffff812e236c>] ? xfs_perag_get+0x80/0xd0
> > [ 3581.189488]  [<ffffffff812e22ec>] ? xfs_check_sizes+0x160/0x160
> > [ 3581.189858]  [<ffffffff812f7831>] ? xfs_inode_ag_iterator+0x6d/0x8f
> > [ 3581.190241]  [<ffffffff812f7398>] ? xfs_sync_worker+0x69/0x69
> > [ 3581.190606]  [<ffffffff812f780b>] xfs_inode_ag_iterator+0x47/0x8f
> > [ 3581.190982]  [<ffffffff811611f5>] ? __sync_filesystem+0x7a/0x7a
> > [ 3581.191352]  [<ffffffff812f7877>] xfs_sync_data+0x24/0x43
> > [ 3581.191703]  [<ffffffff812f7911>] xfs_quiesce_data+0x2c/0x88
> > [ 3581.192065]  [<ffffffff812f5556>] xfs_fs_sync_fs+0x21/0x48
> > [ 3581.192419]  [<ffffffff811611e1>] __sync_filesystem+0x66/0x7a
> > [ 3581.192783]  [<ffffffff8116120b>] sync_one_sb+0x16/0x18
> > [ 3581.193128]  [<ffffffff8113e3e3>] iterate_supers+0x72/0xce
> > [ 3581.193482]  [<ffffffff81161140>] sync_filesystems+0x20/0x22
> > [ 3581.193842]  [<ffffffff8116127e>] sys_sync+0x21/0x33
> > [ 3581.194177]  [<ffffffff819016c2>] system_call_fastpath+0x16/0x1b
> > 
> > --- linux-next.orig/fs/fs-writeback.c	2011-05-02 11:16:50.000000000 +0800
> > +++ linux-next/fs/fs-writeback.c	2011-05-02 11:17:24.000000000 +0800
> > @@ -431,6 +431,15 @@ writeback_single_inode(struct inode *ino
> >  				requeue_io(inode, wb);
> >  			} else {
> >  				/*
> > +				 * sync livelock prevention: each inode is
> > +				 * tagged and synced in one shot. If still
> > +				 * dirty, move it back to s_dirty with updated
> > +				 * dirty time to prevent syncing it again.
> > +				 */
> > +				if (wbc->sync_mode == WB_SYNC_ALL ||
> > +				    wbc->tagged_sync)
> > +					inode->dirtied_when = jiffies;
> > +				/*
> >  				 * Writeback blocked by something other than
> >  				 * congestion. Delay the inode for some time to
> >  				 * avoid spinning on the CPU (100% iowait)
> > 
> > 
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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