[PATCH] for_each_zone macro

Patch from Robert Love.

Attached patch implements for_each_zone(zont_t *) which is a helper
macro to cleanup code of the form:

        for (pgdat = pgdat_list; pgdat; pgdat = pgdat->node_next) {
                for (i = 0; i < MAX_NR_ZONES; ++i) {
                        zone_t * z = pgdat->node_zones + i;
                        /* ... */
                }
        }

and replace it with:

        for_each_zone(zone) {
                /* ... */
        }

This patch only replaces one use of the above loop with the new macro.
Pending code, however, currently in the full rmap patch uses
for_each_zone more extensively.

include/linux/mmzone.h

@@ -177,6 +177,43 @@ extern pg_data_t contig_page_data;
 #define for_each_pgdat(pgdat) \
 	for (pgdat = pgdat_list; pgdat; pgdat = pgdat->pgdat_next)
 
+/*
+ * next_zone - helper magic for for_each_zone()
+ * Thanks to William Lee Irwin III for this piece of ingenuity.
+ */
+static inline zone_t * next_zone(zone_t * zone)
+{
+	pg_data_t *pgdat = zone->zone_pgdat;
+
+	if (zone - pgdat->node_zones < MAX_NR_ZONES - 1)
+		zone++;
+	else if (pgdat->pgdat_next) {
+		pgdat = pgdat->pgdat_next;
+		zone = pgdat->node_zones;
+	} else
+		zone = NULL;
+
+	return zone;
+}
+
+/**
+ * for_each_zone - helper macro to iterate over all memory zones
+ * @zone - pointer to zone_t variable
+ *
+ * The user only needs to declare the zone variable, for_each_zone
+ * fills it in. This basically means for_each_zone() is an
+ * easier to read version of this piece of code:
+ *
+ * for (pgdat = pgdat_list; pgdat; pgdat = pgdat->node_next)
+ * 	for (i = 0; i < MAX_NR_ZONES; ++i) {
+ * 		zone_t * z = pgdat->node_zones + i;
+ * 		...
+ * 	}
+ * }
+ */
+#define for_each_zone(zone) \
+	for (zone = pgdat_list->node_zones; zone; zone = next_zone(zone))
+
 #ifndef CONFIG_DISCONTIGMEM
 
 #define NODE_DATA(nid)		(&contig_page_data)

mm/page_alloc.c

@@ -477,12 +477,11 @@ void free_pages(unsigned long addr, unsigned int order)
  */
 unsigned int nr_free_pages(void)
 {
-	unsigned int i, sum = 0;
-	pg_data_t *pgdat;
+	unsigned int sum = 0;
+	zone_t *zone;
 
-	for (pgdat = pgdat_list; pgdat; pgdat = pgdat->pgdat_next)
-		for (i = 0; i < MAX_NR_ZONES; ++i)
-			sum += pgdat->node_zones[i].free_pages;
+	for_each_zone(zone)
+		sum += zone->free_pages;
 
 	return sum;
 }