[PATCH 0/3] sync livelock fixes v2

[Wu Fengguang <fengguang.wu@intel.com>]

Andrew,

changes from v1:
- retain tagged sync behavior for all WB_SYNC_ALL callers (Dave Chinner)
- rename wbc.for_sync to wbc.tagged_sync
- remove the changes to trace events

The below sync livelock test script is working fine after this patchset:

        sync time: 2
        Dirty:             26492 kB
        Writeback:         30260 kB
        NFS_Unstable:          0 kB
        WritebackTmp:          0 kB
        sync NOT livelocked

In particular patch 2 fixes the sync livelock problem introduced by patch
"writeback: try more writeback as long as something was written".

Thanks,
Fengguang
---

#!/bin/sh

umount /dev/sda7
# mkfs.xfs -f /dev/sda7
mkfs.ext4 /dev/sda7
mount /dev/sda7 /fs

echo $((50<<20)) > /proc/sys/vm/dirty_bytes

pid=
for i in `seq 10`
do     
        dd if=/dev/zero of=/fs/zero-$i bs=1M count=1000 &
        pid="$pid $!"
done

sleep 1

tic=$(date +'%s')
sync
tac=$(date +'%s')

echo
echo sync time: $((tac-tic))
egrep '(Dirty|Writeback|NFS_Unstable)' /proc/meminfo

pidof dd > /dev/null && { kill -9 $pid; echo sync NOT livelocked; }