memory unplug: isolate_lru_page fix

release_pages() in mm/swap.c changes page_count() to be 0 without removing
PageLRU flag...

This means isolate_lru_page() can see a page, PageLRU() &&
page_count(page)==0..  This is BUG.  (get_page() will be called against
count=0 page.)

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -49,9 +49,8 @@ int isolate_lru_page(struct page *page, struct list_head *pagelist)
 		struct zone *zone = page_zone(page);
 
 		spin_lock_irq(&zone->lru_lock);
-		if (PageLRU(page)) {
+		if (PageLRU(page) && get_page_unless_zero(page)) {
 			ret = 0;
-			get_page(page);
 			ClearPageLRU(page);
 			if (PageActive(page))
 				del_page_from_active_list(zone, page);