vmscan: isolate_lru_pages(): stop neighbour search if neighbour cannot be isolated

isolate_lru_pages() does not just isolate LRU tail pages, but also
isolates neighbour pages of the eviction page. The neighbour search does
not stop even if neighbours cannot be isolated which is excessive as the
lumpy reclaim will no longer result in a successful higher order
allocation. This patch stops the PFN neighbour pages if an isolation
fails and moves on to the next block.

Signed-off-by: KOSAKI Motohiro <[e-mail]>
Signed-off-by: Mel Gorman <[e-mail]>
Reviewed-by: Wu Fengguang <[e-mail]>
Reviewed-by: Minchan Kim <[e-mail]>
Reviewed-by: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/vmscan.c

···
 
 			/* Check that we have not crossed a zone boundary. */
 			if (unlikely(page_zone_id(cursor_page) != zone_id))
-				continue;
+				break;
 
 			/*
 			 * If we don't have enough swap space, reclaiming of
···
 			 * pointless.
 			 */
 			if (nr_swap_pages <= 0 && PageAnon(cursor_page) &&
-					!PageSwapCache(cursor_page))
-				continue;
+			    !PageSwapCache(cursor_page))
+				break;
 
 			if (__isolate_lru_page(cursor_page, mode, file) == 0) {
 				list_move(&cursor_page->lru, dst);
···
 					nr_lumpy_dirty++;
 				scan++;
 			} else {
-				if (mode == ISOLATE_BOTH &&
-						page_count(cursor_page))
-					nr_lumpy_failed++;
+				/* the page is freed already. */
+				if (!page_count(cursor_page))
+					continue;
+				break;
 			}
 		}
+
+		/* If we break out of the loop above, lumpy reclaim failed */
+		if (pfn < end_pfn)
+			nr_lumpy_failed++;
 	}
 
 	*scanned = scan;