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=-2.5 required=3.0 tests=MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 94E77ECE596 for ; Tue, 8 Oct 2019 08:47:56 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 639B3205F4 for ; Tue, 8 Oct 2019 08:47:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 639B3205F4 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id EAD3B8E0005; Tue, 8 Oct 2019 04:47:55 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id E36378E0003; Tue, 8 Oct 2019 04:47:55 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D241C8E0005; Tue, 8 Oct 2019 04:47:55 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0229.hostedemail.com [216.40.44.229]) by kanga.kvack.org (Postfix) with ESMTP id A9F3E8E0003 for ; Tue, 8 Oct 2019 04:47:55 -0400 (EDT) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with SMTP id 48FA6180AD803 for ; Tue, 8 Oct 2019 08:47:55 +0000 (UTC) X-FDA: 76019989710.15.wine93_451ab762b002b X-HE-Tag: wine93_451ab762b002b X-Filterd-Recvd-Size: 3550 Received: from mx1.suse.de (mx2.suse.de [195.135.220.15]) by imf35.hostedemail.com (Postfix) with ESMTP for ; Tue, 8 Oct 2019 08:47:54 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id DD2F1AFAE; Tue, 8 Oct 2019 08:47:52 +0000 (UTC) Date: Tue, 8 Oct 2019 10:47:51 +0200 From: Michal Hocko To: Andrew Morton Cc: Qian Cai , tj@kernel.org, vdavydov.dev@gmail.com, hannes@cmpxchg.org, guro@fb.com, cl@linux.com, penberg@kernel.org, rientjes@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects() Message-ID: <20191008084751.GD6681@dhcp22.suse.cz> References: <1570192309-10132-1-git-send-email-cai@lca.pw> <20191004125701.GJ9578@dhcp22.suse.cz> <20191007081621.GE2381@dhcp22.suse.cz> <20191007145902.a1ae6aac11c29d466a445a94@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191007145902.a1ae6aac11c29d466a445a94@linux-foundation.org> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon 07-10-19 14:59:02, Andrew Morton wrote: > On Mon, 7 Oct 2019 10:16:21 +0200 Michal Hocko wrote: > > > On Fri 04-10-19 14:57:01, Michal Hocko wrote: > > > On Fri 04-10-19 08:31:49, Qian Cai wrote: > > > > Long time ago, there fixed a similar deadlock in show_slab_objects() > > > > [1]. However, it is apparently due to the commits like 01fb58bcba63 > > > > ("slab: remove synchronous synchronize_sched() from memcg cache > > > > deactivation path") and 03afc0e25f7f ("slab: get_online_mems for > > > > kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by > > > > just reading files in /sys/kernel/slab which will generate a lockdep > > > > splat below. > > > > > > > > Since the "mem_hotplug_lock" here is only to obtain a stable online node > > > > mask while racing with NUMA node hotplug, in the worst case, the results > > > > may me miscalculated while doing NUMA node hotplug, but they shall be > > > > corrected by later reads of the same files. > > > > > > I think it is important to mention that this doesn't expose the > > > show_slab_objects to use-after-free. There is only a single path that > > > might really race here and that is the slab hotplug notifier callback > > > __kmem_cache_shrink (via slab_mem_going_offline_callback) but that path > > > doesn't really destroy kmem_cache_node data structures. > > Yes, I noted this during review. It's a bit subtle and is worthy of > more than a changelog note, I think. How about this? > > --- a/mm/slub.c~mm-slub-fix-a-deadlock-in-show_slab_objects-fix > +++ a/mm/slub.c > @@ -4851,6 +4851,10 @@ static ssize_t show_slab_objects(struct > * already held which will conflict with an existing lock order: > * > * mem_hotplug_lock->slab_mutex->kernfs_mutex > + * > + * We don't really need mem_hotplug_lock (to hold off > + * slab_mem_going_offline_callback()) here because slab's memory hot > + * unplug code doesn't destroy the kmem_cache->node[] data. > */ Yes please! > #ifdef CONFIG_SLUB_DEBUG > _ > > > Andrew, please add this to the changelog so that we do not have to > > scratch heads again when looking into that code. > > I did that as well. Thanks! -- Michal Hocko SUSE Labs