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Subject[patch 5/6] mm: simplify vmscan vs release refcounting
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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_inc_not_zero 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. This simplifies the handling of the race
because vmscan now *never* touches the refcount of a released page.

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
@@ -286,32 +286,26 @@ 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.
*/

/*
* 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_dec_and_test(&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
===================================================================
--- linux-2.6.orig/mm/vmscan.c
+++ 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;
-
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