mm/compaction: check migrate_pages's return value instead of list_empty()

Many migrate_page's caller check return value instead of list_empy by
cf608ac1 ("mm: compaction: fix COMPACTPAGEFAILED counting").  This patch
makes compaction's migrate_pages consistent with others.  This patch
should not change old behavior.
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -494,12 +494,13 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
 
 	while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {
 		unsigned long nr_migrate, nr_remaining;
+		int err;
 
 		if (!isolate_migratepages(zone, cc))
 			continue;
 
 		nr_migrate = cc->nr_migratepages;
-		migrate_pages(&cc->migratepages, compaction_alloc,
+		err = migrate_pages(&cc->migratepages, compaction_alloc,
 				(unsigned long)cc, false,
 				cc->sync);
 		update_nr_listpages(cc);
@@ -513,7 +514,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
 						nr_remaining);
 
 		/* Release LRU pages not migrated */
-		if (!list_empty(&cc->migratepages)) {
+		if (err) {
 			putback_lru_pages(&cc->migratepages);
 			cc->nr_migratepages = 0;
 		}