mm, page_alloc: remove MIGRATE_RESERVE

MIGRATE_RESERVE preserves an old property of the buddy allocator that
existed prior to fragmentation avoidance -- min_free_kbytes worth of pages
tended to remain contiguous until the only alternative was to fail the
allocation.  At the time it was discovered that high-order atomic
allocations relied on this property so MIGRATE_RESERVE was introduced.  A
later patch will introduce an alternative MIGRATE_HIGHATOMIC so this patch
deletes MIGRATE_RESERVE and supporting code so it'll be easier to review.
Note that this patch in isolation may look like a false regression if
someone was bisecting high-order atomic allocation failures.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vitaly Wool <vitalywool@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -39,8 +39,6 @@ enum {
 	MIGRATE_UNMOVABLE,
 	MIGRATE_MOVABLE,
 	MIGRATE_RECLAIMABLE,
-	MIGRATE_PCPTYPES,	/* the number of types on the pcp lists */
-	MIGRATE_RESERVE = MIGRATE_PCPTYPES,
 #ifdef CONFIG_CMA
 	/*
 	 * MIGRATE_CMA migration type is designed to mimic the way
@@ -63,6 +61,8 @@ enum {
 	MIGRATE_TYPES
 };
 
+#define MIGRATE_PCPTYPES (MIGRATE_RECLAIMABLE+1)
+
 #ifdef CONFIG_CMA
 #  define is_migrate_cma(migratetype) unlikely((migratetype) == MIGRATE_CMA)
 #else
@@ -429,12 +429,6 @@ struct zone {
 
 	const char		*name;
 
-	/*
-	 * Number of MIGRATE_RESERVE page block. To maintain for just
-	 * optimization. Protected by zone->lock.
-	 */
-	int			nr_migrate_reserve_block;
-
 #ifdef CONFIG_MEMORY_ISOLATION
 	/*
 	 * Number of isolated pageblock. It is used to solve incorrect