Compound pages: Use mapcount of first pageto store page order

Use the mapcount field to store the page order. That frees up
lru.prev for other purposes.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/mm.h |    4 ++--
 mm/page_alloc.c    |    1 +
 2 files changed, 3 insertions(+), 2 deletions(-)

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h	2007-09-28 06:33:58.000000000 -0700
+++ linux-2.6/include/linux/mm.h	2007-09-28 06:38:22.000000000 -0700
@@ -355,12 +355,12 @@ static inline int compound_order(struct 
 {
 	if (!PageHead(page))
 		return 0;
-	return (unsigned long)page[1].lru.prev;
+	return atomic_read(&page[1]._mapcount);
 }
 
 static inline void set_compound_order(struct page *page, unsigned long order)
 {
-	page[1].lru.prev = (void *)order;
+	atomic_set(&page[1]._mapcount, order);
 }
 
 /*
Index: linux-2.6/mm/page_alloc.c
===================================================================
--- linux-2.6.orig/mm/page_alloc.c	2007-09-28 06:35:46.000000000 -0700
+++ linux-2.6/mm/page_alloc.c	2007-09-28 06:36:25.000000000 -0700
@@ -269,6 +269,7 @@ static void destroy_compound_page(struct
 	if (unlikely(!PageHead(page)))
 			bad_page(page);
 	__ClearPageHead(page);
+	reset_page_mapcount(page + 1);
 	for (i = 1; i < nr_pages; i++) {
 		struct page *p = page + i;