[patch 1] mm: PG_free flag

[Nick Piggin]

--
SUSE Labs, Novell Inc.
In a future patch we can no longer rely on page_count being stable at any
time, so we can no longer overload PagePrivate && page_count == 0 to mean
the page is free and on the buddy lists.

Index: linux-2.6/include/linux/page-flags.h
===================================================================
--- linux-2.6.orig/include/linux/page-flags.h
+++ linux-2.6/include/linux/page-flags.h
@@ -76,6 +76,8 @@
 #define PG_nosave_free		18	/* Free, should not be written */
 #define PG_uncached		19	/* Page has been mapped as uncached */
 
+#define PG_free			20	/* Page is on the free lists */
+
 /*
  * Global page accounting.  One instance per CPU.  Only unsigned longs are
  * allowed.
@@ -306,6 +308,10 @@ extern void __mod_page_state(unsigned lo
 #define SetPageUncached(page)	set_bit(PG_uncached, &(page)->flags)
 #define ClearPageUncached(page)	clear_bit(PG_uncached, &(page)->flags)
 
+#define PageFree(page)		test_bit(PG_free, &(page)->flags)
+#define __SetPageFree(page)	__set_bit(PG_free, &(page)->flags)
+#define __ClearPageFree(page)	__clear_bit(PG_free, &(page)->flags)
+
 struct page;	/* forward declaration */
 
 int test_clear_page_dirty(struct page *page);
Index: linux-2.6/mm/page_alloc.c
===================================================================
--- linux-2.6.orig/mm/page_alloc.c
+++ linux-2.6/mm/page_alloc.c
@@ -114,7 +114,8 @@ static void bad_page(const char *functio
 			1 << PG_slab    |
 			1 << PG_swapcache |
 			1 << PG_writeback |
-			1 << PG_reserved );
+			1 << PG_reserved |
+			1 << PG_free );
 	set_page_count(page, 0);
 	reset_page_mapcount(page);
 	page->mapping = NULL;
@@ -191,12 +192,12 @@ static inline unsigned long page_order(s
 
 static inline void set_page_order(struct page *page, int order) {
 	page->private = order;
-	__SetPagePrivate(page);
+	__SetPageFree(page);
 }
 
 static inline void rmv_page_order(struct page *page)
 {
-	__ClearPagePrivate(page);
+	__ClearPageFree(page);
 	page->private = 0;
 }
 
@@ -242,9 +243,7 @@ __find_combined_index(unsigned long page
  */
 static inline int page_is_buddy(struct page *page, int order)
 {
-       if (PagePrivate(page)           &&
-           (page_order(page) == order) &&
-            page_count(page) == 0)
+       if (PageFree(page) && (page_order(page) == order))
                return 1;
        return 0;
 }
@@ -327,7 +326,8 @@ static inline void free_pages_check(cons
 			1 << PG_slab	|
 			1 << PG_swapcache |
 			1 << PG_writeback |
-			1 << PG_reserved )))
+			1 << PG_reserved |
+			1 << PG_free )))
 		bad_page(function, page);
 	if (PageDirty(page))
 		__ClearPageDirty(page);
@@ -456,7 +456,8 @@ static void prep_new_page(struct page *p
 			1 << PG_slab    |
 			1 << PG_swapcache |
 			1 << PG_writeback |
-			1 << PG_reserved )))
+			1 << PG_reserved |
+			1 << PG_free )))
 		bad_page(__FUNCTION__, page);
 
 	page->flags &= ~(1 << PG_uptodate | 1 << PG_error |