memory hotplug: make pages from movable zone always isolatable

Pages on movable zone have two types, MIGRATE_MOVABLE and MIGRATE_RESERVE,
both them can be movable, because only movable memory allocation can get
pages from movable zone.  This makes pages in movable zone always be able
to migrate.
Signed-off-by: NShaohua Li <shaohua.li@intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Yakui Zhao <yakui.zhao@intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -4906,13 +4906,16 @@ int set_migratetype_isolate(struct page *page)
 	struct zone *zone;
 	unsigned long flags;
 	int ret = -EBUSY;
+	int zone_idx;
 
 	zone = page_zone(page);
+	zone_idx = zone_idx(zone);
 	spin_lock_irqsave(&zone->lock, flags);
 	/*
 	 * In future, more migrate types will be able to be isolation target.
 	 */
-	if (get_pageblock_migratetype(page) != MIGRATE_MOVABLE)
+	if (get_pageblock_migratetype(page) != MIGRATE_MOVABLE &&
+	    zone_idx != ZONE_MOVABLE)
 		goto out;
 	set_pageblock_migratetype(page, MIGRATE_ISOLATE);
 	move_freepages_block(zone, page, MIGRATE_ISOLATE);