mm: batch-free pcp list if possible

free_pcppages_bulk() frees pages from pcp lists in a round-robin fashion
by keeping batch_free counter.  But it doesn't need to spin if there is
only one non-empty list.  This can be checked by batch_free ==
MIGRATE_PCPTYPES.

[akpm@linux-foundation.org: fix comment]
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -614,6 +614,10 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 			list = &pcp->lists[migratetype];
 		} while (list_empty(list));
 
+		/* This is the only non-empty list. Free them all. */
+		if (batch_free == MIGRATE_PCPTYPES)
+			batch_free = to_free;
+
 		do {
 			page = list_entry(list->prev, struct page, lru);
 			/* must delete as __free_one_page list manipulates */