mm: introduce node_zonelist() for accessing the zonelist for a GFP mask

Introduce a node_zonelist() helper function.  It is used to lookup the
appropriate zonelist given a node and a GFP mask.  The patch on its own is a
cleanup but it helps clarify parts of the two-zonelist-per-node patchset.  If
necessary, it can be merged with the next patch in this set without problems.

Reviewed-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/gfp.h

@@ -154,10 +154,15 @@ static inline enum zone_type gfp_zone(gfp_t flags)
 /*
  * We get the zone list from the current node and the gfp_mask.
  * This zone list contains a maximum of MAXNODES*MAX_NR_ZONES zones.
+ * There are many zonelists per node, two for each active zone.
  *
  * For the normal case of non-DISCONTIGMEM systems the NODE_DATA() gets
  * optimized to &contig_page_data at compile-time.
  */
+static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
+{
+	return NODE_DATA(nid)->node_zonelists + gfp_zone(flags);
+}
 
 #ifndef HAVE_ARCH_FREE_PAGE
 static inline void arch_free_page(struct page *page, int order) { }
@@ -178,8 +183,7 @@ static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
 	if (nid < 0)
 		nid = numa_node_id();
 
-	return __alloc_pages(gfp_mask, order,
-		NODE_DATA(nid)->node_zonelists + gfp_zone(gfp_mask));
+	return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask));
 }
 
 #ifdef CONFIG_NUMA