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

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)



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