[resend][patch 6/6] mm: de-skew page refcounting

Nick Piggin
2006-01-19T19:55:15Z

atomic_add_unless (atomic_inc_not_zero) no longer requires an offset
refcount to function correctly.

Signed-off-by: Nick Piggin <npiggin@suse.de>

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -286,15 +286,6 @@ struct page {
*
* Also, many kernel routines increase the page count before a critical
* routine so they can be sure the page doesn't go away from under them.
- *
- * Since 2.6.6 (approx), a free page has ->_count = -1. This is so that we
- * can use atomic_add_negative(-1, page->_count) to detect when the page
- * becomes free and so that we can also use atomic_inc_and_test to atomically
- * detect when we just tried to grab a ref on a page which some other CPU has
- * already deemed to be freeable.
- *
- * NO code should make assumptions about this internal detail! Use the provided
- * macros which retain the old rules: page_count(page) == 0 is a free page.
*/

/*
@@ -303,8 +294,8 @@ struct page {
*/
static inline int put_page_testzero(struct page *page)
{
- BUG_ON(atomic_read(&page->_count) == -1);
- return atomic_add_negative(-1, &page->_count);
+ BUG_ON(atomic_read(&page->_count) == 0);
+ return atomic_dec_and_test(&page->_count);
}

/*
@@ -313,10 +304,10 @@ static inline int put_page_testzero(stru
*/
static inline int get_page_unless_zero(struct page *page)
{
- return atomic_add_unless(&page->_count, 1, -1);
+ return atomic_inc_not_zero(&page->_count);
}

-#define set_page_count(p,v) atomic_set(&(p)->_count, (v) - 1)
+#define set_page_count(p,v) atomic_set(&(p)->_count, (v))
#define __put_page(p) atomic_dec(&(p)->_count)

extern void FASTCALL(__page_cache_release(struct page *));
@@ -325,7 +316,7 @@ static inline int page_count(struct page
{
if (PageCompound(page))
page = (struct page *)page_private(page);
- return atomic_read(&page->_count) + 1;
+ return atomic_read(&page->_count);
}

static inline void get_page(struct page *page)