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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 BB7D0CA9EC4 for ; Tue, 29 Oct 2019 18:47:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8E21021479 for ; Tue, 29 Oct 2019 18:47:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1572374865; bh=0O3NogRr3ScE1QiLy1fSpxYVQx/SSdKmmMRtJlBQYzQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=NlVFhlKALNtU1LnaapWjjp2xQZAf/qajyL7RsqvHzU+LaNk7Pnm4imJbEm2tVgQUd bZyZVptCYVPRCAPYVex0fFP37dPQqCILySqf6UfLdKGdLNzW3a3HUUxRwYtTrpdXm+ AcFbif9+IzNzJaNJkVnAVDVuwgLHwdXmMvpoyLcU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731878AbfJ2Sro (ORCPT ); Tue, 29 Oct 2019 14:47:44 -0400 Received: from mx2.suse.de ([195.135.220.15]:59344 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729528AbfJ2Sro (ORCPT ); Tue, 29 Oct 2019 14:47:44 -0400 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) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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