[PATCH] mm: simplify vmscan vs release refcounting

The VM has an interesting race where a page refcount can drop to zero, but it
is still on the LRU lists for a short time.  This was solved by testing a 0->1
refcount transition when picking up pages from the LRU, and dropping the
refcount in that case.

Instead, use atomic_add_unless to ensure we never pick up a 0 refcount page
from the LRU, thus a 0 refcount page will never have its refcount elevated
until it is allocated again.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 11 insertions, 8 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 498ff8778fb6..b12d5c76420d 100644
--- a/include/linux/mm.h
+++ b/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(atomic_read(&(p)->_count) == -1);\
-                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)