Messages in this thread Patch in this message | | | From | Nick Piggin <> | Subject | [resend][patch 5/6] mm: simplify vmscan vs release refcounting | Date | Thu, 19 Jan 2006 20:55:05 +0100 (CET) |
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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_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>
Index: linux-2.6/include/linux/mm.h =================================================================== --- linux-2.6.orig/include/linux/mm.h +++ 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 =================================================================== --- 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; - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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