Breakout page_order() to internal.h to avoid special knowledge of the buddy allocator

The statistics patch later needs to know what order a free page is on the free
lists. Rather than having special knowledge of page_private() when
PageBuddy() is set, this patch places out page_order() in internal.h and adds
a VM_BUG_ON to catch using it on non-PageBuddy pages.

Signed-off-by: Mel Gorman <[e-mail]>
Signed-off-by: Christoph Lameter <[e-mail]>
Acked-by: Andy Whitcroft <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/internal.h

···
 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

mm/page_alloc.c

···
 		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);