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 3B06BCA9EAE for ; Tue, 29 Oct 2019 18:47:45 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id F3D73208E3 for ; Tue, 29 Oct 2019 18:47:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F3D73208E3 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 929636B0006; Tue, 29 Oct 2019 14:47:44 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 900376B0007; Tue, 29 Oct 2019 14:47:44 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 83D326B0008; Tue, 29 Oct 2019 14:47:44 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0036.hostedemail.com [216.40.44.36]) by kanga.kvack.org (Postfix) with ESMTP id 6147E6B0006 for ; Tue, 29 Oct 2019 14:47:44 -0400 (EDT) Received: from smtpin27.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id 1B68881DC for ; Tue, 29 Oct 2019 18:47:44 +0000 (UTC) X-FDA: 76097706048.27.dock52_7dad92f49c15e X-HE-Tag: dock52_7dad92f49c15e X-Filterd-Recvd-Size: 4876 Received: from mx1.suse.de (mx2.suse.de [195.135.220.15]) by imf40.hostedemail.com (Postfix) with ESMTP for ; Tue, 29 Oct 2019 18:47:43 +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 96353AFF3; Tue, 29 Oct 2019 18:47:41 +0000 (UTC) Date: Tue, 29 Oct 2019 19:47:39 +0100 From: Michal Hocko To: Shakeel Butt Cc: Roman Gushchin , Johannes Weiner , Andrew Morton , Linux MM , Cgroups , LKML , Eric Dumazet , Greg Thelen , syzbot+13f93c99c06988391efe@syzkaller.appspotmail.com, elver@google.com Subject: Re: [PATCH] mm: memcontrol: fix data race in mem_cgroup_select_victim_node Message-ID: <20191029184739.GP31513@dhcp22.suse.cz> References: <20191029005405.201986-1-shakeelb@google.com> <20191029090347.GG31513@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Tue 29-10-19 11:09:29, Shakeel Butt wrote: > +Marco > > On Tue, Oct 29, 2019 at 2:03 AM Michal Hocko wrote: > > > > On Mon 28-10-19 17:54:05, Shakeel Butt wrote: > > > Syzbot reported the following bug: > > > > > > BUG: KCSAN: data-race in mem_cgroup_select_victim_node / mem_cgroup_select_victim_node > > > > > > write to 0xffff88809fade9b0 of 4 bytes by task 8603 on cpu 0: > > > mem_cgroup_select_victim_node+0xb5/0x3d0 mm/memcontrol.c:1686 > > > try_to_free_mem_cgroup_pages+0x175/0x4c0 mm/vmscan.c:3376 > > > reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349 > > > mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430 > > > tracehook_notify_resume include/linux/tracehook.h:197 [inline] > > > exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163 > > > prepare_exit_to_usermode+0x180/0x1a0 arch/x86/entry/common.c:194 > > > swapgs_restore_regs_and_return_to_usermode+0x0/0x40 > > > > > > read to 0xffff88809fade9b0 of 4 bytes by task 7290 on cpu 1: > > > mem_cgroup_select_victim_node+0x92/0x3d0 mm/memcontrol.c:1675 > > > try_to_free_mem_cgroup_pages+0x175/0x4c0 mm/vmscan.c:3376 > > > reclaim_high.constprop.0+0xf7/0x140 mm/memcontrol.c:2349 > > > mem_cgroup_handle_over_high+0x96/0x180 mm/memcontrol.c:2430 > > > tracehook_notify_resume include/linux/tracehook.h:197 [inline] > > > exit_to_usermode_loop+0x20c/0x2c0 arch/x86/entry/common.c:163 > > > prepare_exit_to_usermode+0x180/0x1a0 arch/x86/entry/common.c:194 > > > swapgs_restore_regs_and_return_to_usermode+0x0/0x40 > > > > > > mem_cgroup_select_victim_node() can be called concurrently which reads > > > and modifies memcg->last_scanned_node without any synchrnonization. So, > > > read and modify memcg->last_scanned_node with READ_ONCE()/WRITE_ONCE() > > > to stop potential reordering. > > > > I am sorry but I do not understand the problem and the fix. Why does the > > race happen and why does _ONCE fixes it? There is still no > > synchronization. Do you want to prevent from memcg->last_scanned_node > > reloading? > > > > The problem is memcg->last_scanned_node can read and modified > concurrently. Though to me it seems like a tolerable race and not > worth to add an explicit lock. Agreed > My aim was to make KCSAN happy here to > look elsewhere for the concurrency bugs. However I see that it might > complain next on memcg->scan_nodes. I would really refrain from adding whatever measure to silence some tool without a deeper understanding of why that is needed. $FOO_ONCE will prevent compiler from making funcy stuff. But this is an int and I would be really surprised if $FOO_ONCE made any practical difference. > Now taking a step back, I am questioning the whole motivation behind > mem_cgroup_select_victim_node(). Since we pass ZONELIST_FALLBACK > zonelist to the reclaimer, the shrink_node will be called for all > potential nodes. Also we don't short circuit the traversal of > shrink_node for all nodes on nr_reclaimed and we scan (size_on_node >> > priority) for all nodes, I don't see the reason behind having round > robin order of node traversal. > > I am thinking of removing the whole mem_cgroup_select_victim_node() > heuristic. Please let me know if there are any objections. I would have to think more about this but this surely sounds like a preferable way than adding $FOO_ONCE to silence the tool. Thanks! -- Michal Hocko SUSE Labs