mm: vmscan: set zone flags before blocking

In shrink_page_list a decision may be made to stall and flag a zone as
ZONE_WRITEBACK so that if a large number of unqueued dirty pages are
encountered later then the reclaimer will stall.  Set ZONE_WRITEBACK
before potentially going to sleep so it is noticed sooner.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Cc: Zlatko Calusic <zcalusic@bitsync.net>
Cc: dormando <dormando@rydia.net>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1445,8 +1445,8 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 	 */
 	if (nr_writeback && nr_writeback >=
 			(nr_taken >> (DEF_PRIORITY - sc->priority))) {
-		wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
 		zone_set_flag(zone, ZONE_WRITEBACK);
+		wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
 	}
 
 	/*