mem-hotplug: call isolate_lru_page with elevated refcount

isolate_lru_page() must be called only with stable reference to page.  So,
let's grab normal page reference.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.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>

mm/memory_hotplug.c

@@ -706,14 +706,15 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 		if (!pfn_valid(pfn))
 			continue;
 		page = pfn_to_page(pfn);
-		if (!page_count(page))
+		if (!get_page_unless_zero(page))
 			continue;
 		/*
 		 * We can skip free pages. And we can only deal with pages on
 		 * LRU.
 		 */
 		ret = isolate_lru_page(page);
 		if (!ret) { /* Success */
+			put_page(page);
 			list_add_tail(&page->lru, &source);
 			move_pages--;
 			inc_zone_page_state(page, NR_ISOLATED_ANON +
@@ -725,6 +726,7 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 			       pfn);
 			dump_page(page);
 #endif
+			put_page(page);
 			/* Because we don't have big zone->lock. we should
 			   check this again here. */
 			if (page_count(page)) {