diff --git a/mm/internal.h b/mm/internal.h
index a3110c0..953f941 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -37,4 +37,14 @@
 extern void fastcall __init __free_pages_bootmem(struct page *page,
 						unsigned int order);
 
+/*
+ * function for dealing with page's order in buddy system.
+ * zone->lock is already acquired when we use these.
+ * So, we don't need atomic page->flags operations here.
+ */
+static inline unsigned long page_order(struct page *page)
+{
+	VM_BUG_ON(!PageBuddy(page));
+	return page_private(page);
+}
 #endif
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a8a53f8..71ced51 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -313,16 +313,6 @@
 		clear_highpage(page + i);
 }
 
-/*
- * function for dealing with page's order in buddy system.
- * zone->lock is already acquired when we use these.
- * So, we don't need atomic page->flags operations here.
- */
-static inline unsigned long page_order(struct page *page)
-{
-	return page_private(page);
-}
-
 static inline void set_page_order(struct page *page, int order)
 {
 	set_page_private(page, order);