mm/zonelist: enumerate zonelists array index

Hardcoding index to zonelists array in gfp_zonelist() is not a good idea,
let's enumerate it to improve readability.

No functional change.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/gfp.h

@@ -385,9 +385,9 @@ static inline enum zone_type gfp_zone(gfp_t flags)
 static inline int gfp_zonelist(gfp_t flags)
 {
 	if (IS_ENABLED(CONFIG_NUMA) && unlikely(flags & __GFP_THISNODE))
-		return 1;
+		return ZONELIST_NOFALLBACK;
 
-	return 0;
+	return ZONELIST_FALLBACK;
 }
 
 /*

include/linux/mmzone.h

@@ -571,19 +571,17 @@ static inline bool zone_is_empty(struct zone *zone)
 /* Maximum number of zones on a zonelist */
 #define MAX_ZONES_PER_ZONELIST (MAX_NUMNODES * MAX_NR_ZONES)
 
+enum {
+	ZONELIST_FALLBACK,	/* zonelist with fallback */
 #ifdef CONFIG_NUMA
-
-/*
- * The NUMA zonelists are doubled because we need zonelists that restrict the
- * allocations to a single node for __GFP_THISNODE.
- *
- * [0]	: Zonelist with fallback
- * [1]	: No fallback (__GFP_THISNODE)
- */
-#define MAX_ZONELISTS 2
-#else
-#define MAX_ZONELISTS 1
+	/*
+	 * The NUMA zonelists are doubled because we need zonelists that restrict
+	 * the allocations to a single node for __GFP_THISNODE.
+	 */
+	ZONELIST_NOFALLBACK,	/* zonelist without fallback (__GFP_THISNODE) */
 #endif
+	MAX_ZONELISTS
+};
 
 /*
  * This struct contains information about a zone in a zonelist. It is stored