Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754963Ab3IYKCR (ORCPT ); Wed, 25 Sep 2013 06:02:17 -0400 Received: from mail-ve0-f175.google.com ([209.85.128.175]:43902 "EHLO mail-ve0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753371Ab3IYKCQ (ORCPT ); Wed, 25 Sep 2013 06:02:16 -0400 MIME-Version: 1.0 In-Reply-To: References: <52118042.30101@oracle.com> <20130819054742.GA28062@bbox> <20130821074939.GE3022@bbox> Date: Wed, 25 Sep 2013 18:02:15 +0800 Message-ID: Subject: Re: [BUG REPORT] ZSWAP: theoretical race condition issues From: Bob Liu To: Weijie Yang Cc: Minchan Kim , Bob Liu , Seth Jennings , Linux-MM , Linux-Kernel Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1854 Lines: 47 On Wed, Sep 25, 2013 at 5:33 PM, Weijie Yang wrote: > 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 Sorry I didn't notice that zswap_frontswap_load() doesn't call rb_erase(). > 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. > Then I think we should erase the entry from rbtree in zswap_frontswap_load(). After the page is decompressed and loaded from zswap, still storing the compressed data in zswap is meanless. -- 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/