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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 EA606C072B5 for ; Fri, 24 May 2019 13:13:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C59F920644 for ; Fri, 24 May 2019 13:13:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391558AbfEXNNy (ORCPT ); Fri, 24 May 2019 09:13:54 -0400 Received: from foss.arm.com ([217.140.101.70]:42714 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2391124AbfEXNNx (ORCPT ); Fri, 24 May 2019 09:13:53 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 1388CA78; Fri, 24 May 2019 06:13:53 -0700 (PDT) Received: from [10.162.42.134] (p8cg001049571a15.blr.arm.com [10.162.42.134]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 99E793F5AF; Fri, 24 May 2019 06:13:49 -0700 (PDT) Subject: Re: mm/compaction: BUG: NULL pointer dereference To: Mel Gorman Cc: Suzuki K Poulose , linux-mm@kvack.org, akpm@linux-foundation.org, mhocko@suse.com, cai@lca.pw, linux-kernel@vger.kernel.org, marc.zyngier@arm.com, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org References: <1558689619-16891-1-git-send-email-suzuki.poulose@arm.com> <20190524123047.GO18914@techsingularity.net> From: Anshuman Khandual Message-ID: <9ae23db2-e696-047b-af18-1e75ebbda085@arm.com> Date: Fri, 24 May 2019 18:43:59 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20190524123047.GO18914@techsingularity.net> Content-Type: text/plain; charset=iso-8859-15 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/24/2019 06:00 PM, Mel Gorman wrote: > On Fri, May 24, 2019 at 04:26:16PM +0530, Anshuman Khandual wrote: >> >> >> On 05/24/2019 02:50 PM, Suzuki K Poulose wrote: >>> Hi, >>> >>> We are hitting NULL pointer dereferences while running stress tests with KVM. >>> See splat [0]. The test is to spawn 100 VMs all doing standard debian >>> installation (Thanks to Marc's automated scripts, available here [1] ). >>> The problem has been reproduced with a better rate of success from 5.1-rc6 >>> onwards. >>> >>> The issue is only reproducible with swapping enabled and the entire >>> memory is used up, when swapping heavily. Also this issue is only reproducible >>> on only one server with 128GB, which has the following memory layout: >>> >>> [32GB@4GB, hole , 96GB@544GB] >>> >>> Here is my non-expert analysis of the issue so far. >>> >>> Under extreme memory pressure, the kswapd could trigger reset_isolation_suitable() >>> to figure out the cached values for migrate/free pfn for a zone, by scanning through >>> the entire zone. On our server it does so in the range of [ 0x10_0000, 0xa00_0000 ], >>> with the following area of holes : [ 0x20_0000, 0x880_0000 ]. >>> In the failing case, we end up setting the cached migrate pfn as : 0x508_0000, which >>> is right in the center of the zone pfn range. i.e ( 0x10_0000 + 0xa00_0000 ) / 2, >>> with reset_migrate = 0x88_4e00, reset_free = 0x10_0000. >>> >>> Now these cached values are used by the fast_isolate_freepages() to find a pfn. However, >>> since we cant find anything during the search we fall back to using the page belonging >>> to the min_pfn (which is the migrate_pfn), without proper checks to see if that is valid >>> PFN or not. This is then passed on to fast_isolate_around() which tries to do : >>> set_pageblock_skip(page) on the page which blows up due to an NULL mem_section pointer. >>> >>> The following patch seems to fix the issue for me, but I am not quite convinced that >>> it is the right fix. Thoughts ? >>> >>> >>> diff --git a/mm/compaction.c b/mm/compaction.c >>> index 9febc8c..9e1b9ac 100644 >>> --- a/mm/compaction.c >>> +++ b/mm/compaction.c >>> @@ -1399,7 +1399,7 @@ fast_isolate_freepages(struct compact_control *cc) >>> page = pfn_to_page(highest); >>> cc->free_pfn = highest; >>> } else { >>> - if (cc->direct_compaction) { >>> + if (cc->direct_compaction && pfn_valid(min_pfn)) { >>> page = pfn_to_page(min_pfn); >> >> pfn_to_online_page() here would be better as it does not add pfn_valid() cost on >> architectures which does not subscribe to CONFIG_HOLES_IN_ZONE. But regardless if >> the compaction is trying to scan pfns in zone holes, then it should be avoided. > > CONFIG_HOLES_IN_ZONE typically applies in special cases where an arch > punches holes within a section. As both do a section lookup, the cost is > similar but pfn_valid in general is less subtle in this case. Normally > pfn_valid_within is only ok when a pfn_valid check has been made on the > max_order aligned range as well as a zone boundary check. In this case, > it's much more straight-forward to leave it as pfn_valid. Sure, makes sense.