mm/writeback: correct dirty page calculation for highmem

ZONE_MOVABLE could be treated as highmem so we need to consider it for
accurate calculation of dirty pages.  And, in following patches,
ZONE_CMA will be introduced and it can be treated as highmem, too.  So,
instead of manually adding stat of ZONE_MOVABLE, looping all zones and
check whether the zone is highmem or not and add stat of the zone which
can be treated as highmem.

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@suse.de>
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/page-writeback.c

@@ -296,11 +296,15 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
 #ifdef CONFIG_HIGHMEM
 	int node;
 	unsigned long x = 0;
+	int i;
 
 	for_each_node_state(node, N_HIGH_MEMORY) {
-		struct zone *z = &NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
+		for (i = 0; i < MAX_NR_ZONES; i++) {
+			struct zone *z = &NODE_DATA(node)->node_zones[i];
 
-		x += zone_dirtyable_memory(z);
+			if (is_highmem(z))
+				x += zone_dirtyable_memory(z);
+		}
 	}
 	/*
 	 * Unreclaimable memory (kernel memory or anonymous memory