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=-0.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,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 E5CB9C83000 for ; Thu, 30 Apr 2020 01:16:30 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id A5E6C20757 for ; Thu, 30 Apr 2020 01:16:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chrisdown.name header.i=@chrisdown.name header.b="NEm5AmoT" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A5E6C20757 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=chrisdown.name Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 3CEE68E0005; Wed, 29 Apr 2020 21:16:30 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 37F738E0001; Wed, 29 Apr 2020 21:16:30 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2BC3B8E0005; Wed, 29 Apr 2020 21:16:30 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0245.hostedemail.com [216.40.44.245]) by kanga.kvack.org (Postfix) with ESMTP id 11EBE8E0001 for ; Wed, 29 Apr 2020 21:16:30 -0400 (EDT) Received: from smtpin03.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id AD1854FEC for ; Thu, 30 Apr 2020 01:16:29 +0000 (UTC) X-FDA: 76762756098.03.roll11_38ab2afdfce02 X-HE-Tag: roll11_38ab2afdfce02 X-Filterd-Recvd-Size: 4053 Received: from mail-wm1-f68.google.com (mail-wm1-f68.google.com [209.85.128.68]) by imf38.hostedemail.com (Postfix) with ESMTP for ; Thu, 30 Apr 2020 01:16:29 +0000 (UTC) Received: by mail-wm1-f68.google.com with SMTP id u16so47130wmc.5 for ; Wed, 29 Apr 2020 18:16:29 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chrisdown.name; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=l64V23ovWLJzXgRwUJUSoRHmFCYvD/C61lUFlvHuAM8=; b=NEm5AmoTD2jKzz4hSccJVFQ1FPjZ6Bxkvy9pbF1n13au2XvE7BgLVefCXG75LJARlN vnoy8WuPgSjgJpXQR+RcXTzwoRRtfu4SDZm+Yc+p6DSFyw0qj5XWcrtNba7s9fwH42iU Dmrs/6XTu4+ArHUN5ftQeDPkUfs2GVxihVugU= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=l64V23ovWLJzXgRwUJUSoRHmFCYvD/C61lUFlvHuAM8=; b=KaBs4osB0aI4bbfpoAhwG1T6iT1dEDzwV+23DhCrC03D2qbboISIIwnqQevQzClK/N V+MXl+m2OxvSdQonpcUYWhe8Zwwh+HsRANYQg+efOkm+fa77wVYGiZP9458RK1/3xtiB +L5B6HDQfe2WOSDHJnTj+Pk9NP9PnuzS8U1mL5bQ9sOSa+5zuS+f6A+WoQDOaQn8azDb yXOWLq9jKbccSxWZ+lw5wgdHFozO2n9YtyMckAu9mY3DO8RxZPjrYR5pLJcnOvQhvtKt /Ga6w7Xct8ad42pHOUBnxL8VUJpo+qYyHC3QZsfIkbnF2/mf24sEjCQducB/IyIRoFDF ug8Q== X-Gm-Message-State: AGi0PuYOgVaOToeHVZaJgYh7/YpSI+qQB7IDn9FtwiqpqLE6uveCeE1d bKpyWlRlYiTl8VNfk3XaZ9Lj1Q== X-Google-Smtp-Source: APiQypLqV+ArzORPofbv6BQwM8YI/oPdsz+Copu8gSwvyT8oSBSXG3Ri7l1nKh86jTZCo5uAEF/Ejw== X-Received: by 2002:a05:600c:2c04:: with SMTP id q4mr93056wmg.7.1588209387919; Wed, 29 Apr 2020 18:16:27 -0700 (PDT) Received: from localhost ([2a01:4b00:8432:8a00:56e1:adff:fe3f:49ed]) by smtp.gmail.com with ESMTPSA id e11sm1467438wrn.87.2020.04.29.18.16.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Apr 2020 18:16:27 -0700 (PDT) Date: Thu, 30 Apr 2020 02:16:26 +0100 From: Chris Down To: Yafang Shao Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Linux MM , Cgroups , LKML Subject: Re: [PATCH 1/2] mm, memcg: Avoid stale protection values when cgroup is above protection Message-ID: <20200430011626.GA2754277@chrisdown.name> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: 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: Hi Yafang, Yafang Shao writes: >Would you pls. add some comments above these newly added WRITE_ONCE() ? >E.g. >What does them mean to fix ? >Why do we must add WRITE_ONCE() and READ_ONCE here and there all over >the memcg protection ? >Otherwise, it may be harder to understand by the others. There is already discussion in the changelogs for previous store tear improvements. For example, b3a7822e5e75 ("mm, memcg: prevent mem_cgroup_protected store tearing"). WRITE_ONCE and READ_ONCE are standard compiler barriers, in this case, to avoid store tears from writes in another thread (effective protection caching is designed by its very nature to permit racing, but tearing is non-ideal). You can find out more about them in the "COMPILER BARRIER" section in Documentation/memory-barriers.txt. I'm not really seeing the value of adding an extra comment about this specific use of them, unless you have some more explicit concern.