mm: simplify vmscan vs release refcounting

Index: linux-2.6/include/linux/mm.h
===================================================================
@@ -301,17 +301,20 @@ struct page {
  * Drop a ref, return true if the logical refcount fell to zero (the page has
  * no users)
  */
-#define put_page_testzero(p)				\
-	({						\
-		BUG_ON(page_count(p) == 0);		\
-		atomic_add_negative(-1, &(p)->_count);	\
-	})
+static inline int put_page_testzero(struct page *page)
+{
+	BUG_ON(atomic_read(&page->_count) == -1);
+	return atomic_add_negative(-1, &page->_count);
+}
 
 /*
- * Grab a ref, return true if the page previously had a logical refcount of
- * zero.  ie: returns true if we just grabbed an already-deemed-to-be-free page
+ * Try to grab a ref unless the page has a refcount of zero, return false if
+ * that is the case.
  */
-#define get_page_testone(p)	atomic_inc_and_test(&(p)->_count)
+static inline int get_page_unless_zero(struct page *page)
+{
+	return atomic_add_unless(&page->_count, 1, -1);
+}
 
 #define set_page_count(p,v) 	atomic_set(&(p)->_count, (v) - 1)
 #define __put_page(p)		atomic_dec(&(p)->_count)

Index: linux-2.6/mm/vmscan.c
===================================================================
@@ -821,29 +821,26 @@ static int isolate_lru_pages(int nr_to_s
 	int scan = 0;
 
 	while (scan++ < nr_to_scan && !list_empty(src)) {
+		struct list_head *target;
 		page = lru_to_page(src);
 		prefetchw_prev_lru_page(page, src, flags);
 
 		BUG_ON(!PageLRU(page));
 
 		list_del(&page->lru);
-		if (unlikely(get_page_testone(page))) {
+		target = src;
+		if (likely(get_page_unless_zero(page))) {
 			/*
-			 * It is being freed elsewhere
+			 * Be careful not to clear PageLRU until after we're
+			 * sure the page is not being freed elsewhere -- the
+			 * page release code relies on it.
 			 */
-			__put_page(page);
-			list_add(&page->lru, src);
-			continue;
-		}
+			ClearPageLRU(page);
+			target = dst;
+			nr_taken++;
+		} /* else it is being freed elsewhere */
 
-		/*
-		 * Be careful not to clear PageLRU until after we're sure
-		 * the page is not being freed elsewhere -- the page release
-		 * code relies on it.
-		 */
-		ClearPageLRU(page);
-		list_add(&page->lru, dst);
-		nr_taken++;
+		list_add(&page->lru, target);
 	}
 
 	*scanned = scan;

Index: 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)