Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965080AbbEMBVQ (ORCPT ); Tue, 12 May 2015 21:21:16 -0400 Received: from mail-pa0-f42.google.com ([209.85.220.42]:34202 "EHLO mail-pa0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964912AbbEMBVP (ORCPT ); Tue, 12 May 2015 21:21:15 -0400 Date: Wed, 13 May 2015 10:21:07 +0900 From: Minchan Kim To: Andrew Morton Cc: Vladimir Davydov , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Paul E. McKenney" , "Kirill A. Shutemov" , Rik van Riel , Hugh Dickins Subject: Re: [PATCH v2] rmap: fix theoretical race between do_wp_page and shrink_active_list Message-ID: <20150513012106.GB8267@blaptop> References: <1431425919-28057-1-git-send-email-vdavydov@parallels.com> <20150512152840.20805775ae82c69b9a8f3028@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150512152840.20805775ae82c69b9a8f3028@linux-foundation.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3215 Lines: 80 Hello Andrew, On Tue, May 12, 2015 at 03:28:40PM -0700, Andrew Morton wrote: > On Tue, 12 May 2015 13:18:39 +0300 Vladimir Davydov wrote: > > > 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. > > > > ... > > > > --- 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); > > Please let's not put things like WRITE_ONCE() in there without > documenting them - otherwise it's terribly hard for readers to work out > why it was added. > > How's this look? > > --- a/mm/rmap.c~rmap-fix-theoretical-race-between-do_wp_page-and-shrink_active_list-fix > +++ a/mm/rmap.c > @@ -950,6 +950,11 @@ void page_move_anon_rmap(struct page *pa > VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page); > > anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON; > + /* > + * Ensure that anon_vma and the PAGE_MAPPING_ANON bit are written > + * simultaneously, so a concurrent reader (eg shrink_active_list) will IMHO, rather than shrink_active_list, PageAnon in page_referenced is better to me. -- Kind regards, Minchan Kim -- 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/