vmscan: fix zone shrinking exit when scan work is done

Commit 3e7d344 ("mm: vmscan: reclaim order-0 and use compaction
instead of lumpy reclaim") introduced an indefinite loop in
shrink_zone().

It meant to break out of this loop when no pages had been reclaimed and
not a single page was even scanned.  The way it would detect the latter
is by taking a snapshot of sc->nr_scanned at the beginning of the
function and comparing it against the new sc->nr_scanned after the scan
loop.  But it would re-iterate without updating that snapshot, looping
forever if sc->nr_scanned changed at least once since shrink_zone() was
invoked.

This is not the sole condition that would exit that loop, but it
requires other processes to change the zone state, as the reclaimer that
is stuck obviously can not anymore.

This is only happening for higher-order allocations, where reclaim is
run back to back with compaction.

Signed-off-by: Johannes Weiner <[email protected]>
Reported-by: Michal Hocko <[email protected]>
Tested-by: Kent Overstreet<[email protected]>
Reported-by: Kent Overstreet <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Rik van Riel <[email protected]>
Reviewed-by: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/vmscan.c

@@ -1882,12 +1882,12 @@ static void shrink_zone(int priority, struct zone *zone,
 	unsigned long nr[NR_LRU_LISTS];
 	unsigned long nr_to_scan;
 	enum lru_list l;
-	unsigned long nr_reclaimed;
+	unsigned long nr_reclaimed, nr_scanned;
 	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
-	unsigned long nr_scanned = sc->nr_scanned;
 
 restart:
 	nr_reclaimed = 0;
+	nr_scanned = sc->nr_scanned;
 	get_scan_count(zone, sc, nr, priority);
 
 	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||