mm: use global_dirty_limit in throttle_vm_writeout()

When starting a memory hog task, a desktop box w/o swap is found to go
unresponsive for a long time.  It's solely caused by lots of congestion
waits in throttle_vm_writeout():

 gnome-system-mo-4201 553.073384: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
 gnome-system-mo-4201 553.073386: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000
           gtali-4237 553.080377: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
           gtali-4237 553.080378: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000
            Xorg-3483 553.103375: congestion_wait: throttle_vm_writeout+0x70/0x7f shrink_mem_cgroup_zone+0x48f/0x4a1
            Xorg-3483 553.103377: writeback_congestion_wait: usec_timeout=100000 usec_delayed=100000

The root cause is, the dirty threshold is knocked down a lot by the memory
hog task.  Fixed by using global_dirty_limit which decreases gradually on
such events and can guarantee we stay above (the also decreasing) nr_dirty
in the progress of following down to the new dirty threshold.

Signed-off-by: Fengguang Wu <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Ying Han <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page-writeback.c

@@ -1472,6 +1472,7 @@ void throttle_vm_writeout(gfp_t gfp_mask)
 
         for ( ; ; ) {
 		global_dirty_limits(&background_thresh, &dirty_thresh);
+		dirty_thresh = hard_dirty_limit(dirty_thresh);
 
                 /*
                  * Boost the allowable dirty threshold a bit for page