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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 91AB4C0044C for ; Wed, 7 Nov 2018 08:50:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5CF212081D for ; Wed, 7 Nov 2018 08:50:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5CF212081D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729859AbeKGSTf (ORCPT ); Wed, 7 Nov 2018 13:19:35 -0500 Received: from mx2.suse.de ([195.135.220.15]:37160 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726315AbeKGSTf (ORCPT ); Wed, 7 Nov 2018 13:19:35 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 0477CAE24; Wed, 7 Nov 2018 08:50:11 +0000 (UTC) Subject: Re: [PATCH v1 0/4]mm: convert totalram_pages, totalhigh_pages and managed pages to atomic To: Konstantin Khlebnikov , Arun KS Cc: keescook@chromium.org, minchan@kernel.org, getarunks@gmail.com, gregkh@linuxfoundation.org, akpm@linux-foundation.org, mhocko@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, julia.lawall@lip6.fr References: <1540551662-26458-1-git-send-email-arunks@codeaurora.org> <9b210d4cc9925caf291412d7d45f16d7@codeaurora.org> <63d9f48c-e39f-d345-0fb6-2f04afe769a2@yandex-team.ru> <08a61c003eed0280fd82f6200debcbca@codeaurora.org> <10c88df6-dbb1-7490-628c-055d59b5ad8e@yandex-team.ru> <22fa2222012341a54f6b0b6aea341aa2@codeaurora.org> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 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 11/7/18 8:02 AM, Konstantin Khlebnikov wrote: > On 06.11.2018 11:43, Arun KS wrote: >> On 2018-11-06 14:07, Konstantin Khlebnikov wrote: >>> On 06.11.2018 11:30, Arun KS wrote: >>>> On 2018-11-06 13:47, Konstantin Khlebnikov wrote: >>>>> On 06.11.2018 8:38, Arun KS wrote: >>>>>> Any comments? >>>>> >>>>> Looks good. >>>>> Except unclear motivation behind this change. >>>>> This should be in comment of one of patch. >>>> >>>> totalram_pages, zone->managed_pages and totalhigh_pages are sometimes modified outside managed_page_count_lock. Hence convert these >>>> variable to atomic to avoid readers potentially seeing a store tear. >>> >>> So, this is just theoretical issue or splat from sanitizer. >>> After boot memory online\offline are strictly serialized by rw-semaphore. >> >> Few instances which can race with hot add. Please see below, >> https://patchwork.kernel.org/patch/10627521/ > Could you point what exactly are you fixing with this set? > > from v2: > > > totalram_pages, zone->managed_pages and totalhigh_pages updates > > are protected by managed_page_count_lock, but readers never care > > about it. Convert these variables to atomic to avoid readers > > potentially seeing a store tear. > > This? > > > Aligned unsigned long almost always stored at once. The point is "almost always", so better not rely on it :) But the main motivation was that managed_page_count_lock handling was complicating Arun's "memory_hotplug: Free pages as higher order" patch and it seemed a better idea to just remove and convert this to atomics, with preventing potential store-to-read tearing as a bonus. It would be nice to mention it in the changelogs though. > To make it completely correct you could replace > > a += b; > > with > > WRITE_ONCE(a, a + b); Wouldn't be enough to get rid of the locks.