memory hotplug: fix next block calculation in is_removable

next_active_pageblock() is for finding next _used_ freeblock.  It skips
several blocks when it finds there are a chunk of free pages lager than
pageblock.  But it has 2 bugs.

  1. We have no lock. page_order(page) - pageblock_order can be minus.
  2. pageblocks_stride += is wrong. it should skip page_order(p) of pages.
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -584,19 +584,19 @@ static inline int pageblock_free(struct page *page)
 /* Return the start of the next active pageblock after a given page */
 static struct page *next_active_pageblock(struct page *page)
 {
-	int pageblocks_stride;
-
 	/* Ensure the starting page is pageblock-aligned */
 	BUG_ON(page_to_pfn(page) & (pageblock_nr_pages - 1));
 
-	/* Move forward by at least 1 * pageblock_nr_pages */
-	pageblocks_stride = 1;
-
 	/* If the entire pageblock is free, move to the end of free page */
-	if (pageblock_free(page))
-		pageblocks_stride += page_order(page) - pageblock_order;
+	if (pageblock_free(page)) {
+		int order;
+		/* be careful. we don't have locks, page_order can be changed.*/
+		order = page_order(page);
+		if ((order < MAX_ORDER) && (order >= pageblock_order))
+			return page + (1 << order);
+	}
 
-	return page + (pageblocks_stride * pageblock_nr_pages);
+	return page + pageblock_nr_pages;
 }
 
 /* Checks if this range of memory is likely to be hot-removable. */