compound pages: Store address in page[1]

Storing the address in page[1].lru.prev allows an optimization of the
address determination for compund pages. Storing the address in the
page structs of the compound page itself is going to be important for
virtual compound pages.

Add a new function

	page_to_addr

in order to determine the address of a page that can exploit the address
stored in the compound page.

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

---
 include/linux/mm.h |   12 ++++++++++++
 mm/page_alloc.c    |    9 +++++++++
 2 files changed, 21 insertions(+)

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h	2007-09-28 09:54:34.000000000 -0700
+++ linux-2.6/include/linux/mm.h	2007-09-28 09:54:58.000000000 -0700
@@ -363,6 +363,18 @@ static inline void set_compound_order(st
 	atomic_set(&page[1]._mapcount, order);
 }
 
+extern void *page_to_addr(struct page *page);
+
+static inline void *compound_address(struct page *page)
+{
+	return (void *)page[1].lru.prev;
+}
+
+static inline void set_compound_address(struct page *page, void *addr)
+{
+	page[1].lru.prev = addr;
+}
+
 /*
  * Multiple processes may "see" the same page. E.g. for untouched
  * mappings of /dev/null, all processes see the same page full of
Index: linux-2.6/mm/page_alloc.c
===================================================================
--- linux-2.6.orig/mm/page_alloc.c	2007-09-28 09:54:34.000000000 -0700
+++ linux-2.6/mm/page_alloc.c	2007-09-28 09:54:48.000000000 -0700
@@ -242,6 +242,14 @@ static void free_compound_page(struct pa
 	__free_pages_ok(page, compound_order(page));
 }
 
+void *page_to_addr(struct page *page)
+{
+	if (likely(!PageHead(page)))
+		return page_address(page);
+	return compound_address(page);
+}
+EXPORT_SYMBOL(page_to_addr);
+
 static void prep_compound_page(struct page *page, unsigned long order)
 {
 	int i;
@@ -249,6 +257,7 @@ static void prep_compound_page(struct pa
 
 	set_compound_page_dtor(page, free_compound_page);
 	set_compound_order(page, order);
+	set_compound_address(page, page_address(page));
 	__SetPageHead(page);
 	for (i = 1; i < nr_pages; i++) {
 		struct page *p = page + i;