From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8FCA3C4346E for ; Sun, 27 Sep 2020 05:33:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3F1E123A40 for ; Sun, 27 Sep 2020 05:33:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730113AbgI0Fd1 (ORCPT ); Sun, 27 Sep 2020 01:33:27 -0400 Received: from mga02.intel.com ([134.134.136.20]:4823 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726382AbgI0Fd1 (ORCPT ); Sun, 27 Sep 2020 01:33:27 -0400 IronPort-SDR: r/6ggePXmMXt/tkZDIRuUjUGr6CvCHMWJ6QZZBOREUR8Cyb7NBF9mhG6O/yrSQG0E0iZlgOZwH GVWdgJ1EYdxQ== X-IronPort-AV: E=McAfee;i="6000,8403,9756"; a="149503058" X-IronPort-AV: E=Sophos;i="5.77,308,1596524400"; d="scan'208";a="149503058" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 Sep 2020 22:33:24 -0700 IronPort-SDR: KZ5U8p5UglqFP5AJsGeZrD3idzYlR1g4EWZKnHx9q1UnfgQiK7KtvaWpQ1klgZbBiUBXz+QNPR YmpMu5TvaVnA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.77,308,1596524400"; d="scan'208";a="488122447" Received: from yhuang-dev.sh.intel.com (HELO yhuang-dev) ([10.239.159.65]) by orsmga005.jf.intel.com with ESMTP; 26 Sep 2020 22:33:22 -0700 From: "Huang\, Ying" To: Rafael Aquini Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org Subject: Re: [PATCH] mm: swapfile: avoid split_swap_cluster() NULL pointer dereference References: <20200923043459.GL795820@optiplex-lnx> <87sgb9oz1u.fsf@yhuang-dev.intel.com> <20200923130138.GM795820@optiplex-lnx> <87blhwng5f.fsf@yhuang-dev.intel.com> <20200924020928.GC1023012@optiplex-lnx> <877dsjessq.fsf@yhuang-dev.intel.com> <20200924063038.GD1023012@optiplex-lnx> <87tuvnd3db.fsf@yhuang-dev.intel.com> <20200924150833.GE1023012@optiplex-lnx> <87r1qqbkx5.fsf@yhuang-dev.intel.com> <20200926151643.GA1325930@optiplex-lnx> Date: Sun, 27 Sep 2020 13:33:21 +0800 In-Reply-To: <20200926151643.GA1325930@optiplex-lnx> (Rafael Aquini's message of "Sat, 26 Sep 2020 11:16:43 -0400") Message-ID: <874knjbx7i.fsf@yhuang-dev.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Rafael Aquini writes: > On Fri, Sep 25, 2020 at 11:21:58AM +0800, Huang, Ying wrote: >> Rafael Aquini writes: >> >> Or, can you help to run the test with a debug kernel based on upstream >> >> kernel. I can provide some debug patch. >> >> >> > >> > Sure, I can set your patches to run with the test cases we have that tend to >> > reproduce the issue with some degree of success. >> >> Thanks! >> >> I found a race condition. During THP splitting, "head" may be unlocked >> before calling split_swap_cluster(), because head != page during >> deferred splitting. So we should call split_swap_cluster() before >> unlocking. The debug patch to do that is as below. Can you help to >> test it? >> > > > I finally could grab a good crashdump and confirm that head is really > not locked. Thanks! That's really helpful for us to root cause the bug. > I still need to dig into it to figure out more about the > crash. I guess that your patch will guarantee that lock on head, but > it still doesn't help on explaining how did we get the THP marked as > PG_swapcache, given that it should fail add_to_swap()->get_swap_page() > right? Because ClearPageCompound(head) is called in __split_huge_page(), then all subpages except "page" are unlocked. So previously, when split_swap_cluster() is called in split_huge_page_to_list(), the THP has been split already and "head" may be unlocked. Then the normal page "head" can be added to swap cache. CPU1 CPU2 ---- ---- deferred_split_scan() split_huge_page(page) /* page isn't compound head */ split_huge_page_to_list(page, NULL) __split_huge_page(page, ) ClearPageCompound(head) /* unlock all subpages except page (not head) */ add_to_swap(head) /* not THP */ get_swap_page(head) add_to_swap_cache(head, ) SetPageSwapCache(head) if PageSwapCache(head) split_swap_cluster(/* swap entry of head */) /* Deref sis->cluster_info: NULL accessing! */ > I'll give your patch a run over the weekend, hopefully we'll have more > info on this next week. Thanks! Best Regards, Huang, Ying >> Best Regards, >> Huang, Ying >> >> ------------------------8<---------------------------- >> From 24ce0736a9f587d2dba12f12491c88d3e296a491 Mon Sep 17 00:00:00 2001 >> From: Huang Ying >> Date: Fri, 25 Sep 2020 11:10:56 +0800 >> Subject: [PATCH] dbg: Call split_swap_clsuter() before unlock page during >> split THP >> >> --- >> mm/huge_memory.c | 13 +++++++------ >> 1 file changed, 7 insertions(+), 6 deletions(-) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index faadc449cca5..8d79e5e6b46e 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -2444,6 +2444,12 @@ static void __split_huge_page(struct page *page, struct list_head *list, >> >> remap_page(head); >> >> + if (PageSwapCache(head)) { >> + swp_entry_t entry = { .val = page_private(head) }; >> + >> + split_swap_cluster(entry); >> + } >> + >> for (i = 0; i < HPAGE_PMD_NR; i++) { >> struct page *subpage = head + i; >> if (subpage == page) >> @@ -2678,12 +2684,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) >> } >> >> __split_huge_page(page, list, end, flags); >> - if (PageSwapCache(head)) { >> - swp_entry_t entry = { .val = page_private(head) }; >> - >> - ret = split_swap_cluster(entry); >> - } else >> - ret = 0; >> + ret = 0; >> } else { >> if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) { >> pr_alert("total_mapcount: %u, page_count(): %u\n", >> -- >> 2.28.0 >>