Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754949Ab3IYJdq (ORCPT ); Wed, 25 Sep 2013 05:33:46 -0400 Received: from mail-ie0-f181.google.com ([209.85.223.181]:53142 "EHLO mail-ie0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754880Ab3IYJdo (ORCPT ); Wed, 25 Sep 2013 05:33:44 -0400 MIME-Version: 1.0 In-Reply-To: References: <52118042.30101@oracle.com> <20130819054742.GA28062@bbox> <20130821074939.GE3022@bbox> Date: Wed, 25 Sep 2013 17:33:43 +0800 Message-ID: Subject: Re: [BUG REPORT] ZSWAP: theoretical race condition issues From: Weijie Yang To: Bob Liu Cc: Minchan Kim , Bob Liu , Seth Jennings , Linux-MM , Linux-Kernel Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1814 Lines: 47 On Wed, Sep 25, 2013 at 4:31 PM, Bob Liu wrote: > On Wed, Sep 25, 2013 at 4:09 PM, Weijie Yang wrote: >> I think I find a new issue, for integrity of this mail thread, I reply >> to this mail. >> >> It is a concurrence issue either, when duplicate store and reclaim >> concurrentlly. >> >> zswap entry x with offset A is already stored in zswap backend. >> Consider the following scenario: >> >> thread 0: reclaim entry x (get refcount, but not call zswap_get_swap_cache_page) >> >> thread 1: store new page with the same offset A, alloc a new zswap entry y. >> store finished. shrink_page_list() call __remove_mapping(), and now >> it is not in swap_cache >> > > But I don't think swap layer will call zswap with the same offset A. 1. store page of offset A in zswap 2. some time later, pagefault occur, load page data from zswap. But notice that zswap entry x is still in zswap because it is not frontswap_tmem_exclusive_gets_enabled. this page is with PageSwapCache(page) and page_private(page) = entry.val 3. change this page data, and it become dirty 4. some time later again, swap this page on the same offset A. so, a duplicate store happens. what I can think is that use flags and CAS to protect store and reclaim on the same offset happens concurrentlly. >> thread 0: zswap_get_swap_cache_page called. old page data is added to swap_cache >> >> Now, swap cache has old data rather than new data for offset A. >> error will happen If do_swap_page() get page from swap_cache. >> > > -- > Regards, > --Bob -- 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/