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

[Jan Kara <jack@suse.cz>]

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. 

								Honza
> 
> CC: Jan Kara <jack@suse.cz>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.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@suse.cz>
SUSE Labs, CR