mm/highmem: make nr_free_highpages() handles all highmem zones by itself

nr_free_highpages() manually adds statistics per each highmem zone and
returns a total value for them.  Whenever we add a new highmem zone, we
need to consider this function and it's really troublesome.  Make it
handle all highmem zones by itself.
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Laura Abbott <lauraa@codeaurora.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/highmem.c

@@ -112,16 +112,12 @@ EXPORT_PER_CPU_SYMBOL(__kmap_atomic_idx);
 
 unsigned int nr_free_highpages (void)
 {
-	pg_data_t *pgdat;
+	struct zone *zone;
 	unsigned int pages = 0;
 
-	for_each_online_pgdat(pgdat) {
-		pages += zone_page_state(&pgdat->node_zones[ZONE_HIGHMEM],
-			NR_FREE_PAGES);
-		if (zone_movable_is_highmem())
-			pages += zone_page_state(
-					&pgdat->node_zones[ZONE_MOVABLE],
-					NR_FREE_PAGES);
+	for_each_populated_zone(zone) {
+		if (is_highmem(zone))
+			pages += zone_page_state(zone, NR_FREE_PAGES);
 	}
 
 	return pages;