Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752962AbbELKSy (ORCPT ); Tue, 12 May 2015 06:18:54 -0400 Received: from mx2.parallels.com ([199.115.105.18]:60352 "EHLO mx2.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752673AbbELKSw (ORCPT ); Tue, 12 May 2015 06:18:52 -0400 From: Vladimir Davydov To: Andrew Morton CC: , , "Paul E. McKenney" , "Kirill A. Shutemov" , Rik van Riel , Hugh Dickins Subject: [PATCH v2] rmap: fix theoretical race between do_wp_page and shrink_active_list Date: Tue, 12 May 2015 13:18:39 +0300 Message-ID: <1431425919-28057-1-git-send-email-vdavydov@parallels.com> X-Mailer: git-send-email 1.7.10.4 MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2626 Lines: 72 As noted by Paul the compiler is free to store a temporary result in a variable on stack, heap or global unless it is explicitly marked as volatile, see: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html#sample-optimizations This can result in a race between do_wp_page() and shrink_active_list() as follows. In do_wp_page() we can call page_move_anon_rmap(), which sets page->mapping as follows: anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; page->mapping = (struct address_space *) anon_vma; The page in question may be on an LRU list, because nowhere in do_wp_page() we remove it from the list, neither do we take any LRU related locks. Although the page is locked, shrink_active_list() can still call page_referenced() on it concurrently, because the latter does not require an anonymous page to be locked: CPU0 CPU1 ---- ---- do_wp_page shrink_active_list lock_page page_referenced PageAnon->yes, so skip trylock_page page_move_anon_rmap page->mapping = anon_vma rmap_walk PageAnon->no rmap_walk_file BUG page->mapping += PAGE_MAPPING_ANON This patch fixes this race by explicitly forbidding the compiler to split page->mapping store in page_move_anon_rmap() with the aid of WRITE_ONCE. Signed-off-by: Vladimir Davydov Cc: "Paul E. McKenney" Cc: "Kirill A. Shutemov" Cc: Rik van Riel Cc: Hugh Dickins --- Changes in v2: - do not add READ_ONCE to PageAnon and WRITE_ONCE to __page_set_anon_rmap and __hugepage_set_anon_rmap (Kirill) mm/rmap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/rmap.c b/mm/rmap.c index 24dd3f9fee27..8b18fd4227d1 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -950,7 +950,7 @@ void page_move_anon_rmap(struct page *page, VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page); anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; - page->mapping = (struct address_space *) anon_vma; + WRITE_ONCE(page->mapping, (struct address_space *) anon_vma); } /** -- 1.7.10.4 -- 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/