[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>
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8e477b1..e21bab4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1083,29 +1083,26 @@
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;
