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 BB7D4C0044C for ; Wed, 7 Nov 2018 08:44:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 833B02081D for ; Wed, 7 Nov 2018 08:44:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 833B02081D 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 S1730484AbeKGSNY (ORCPT ); Wed, 7 Nov 2018 13:13:24 -0500 Received: from mx2.suse.de ([195.135.220.15]:35954 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726223AbeKGSNX (ORCPT ); Wed, 7 Nov 2018 13:13:23 -0500 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 A7BE2ACC6; Wed, 7 Nov 2018 08:44:00 +0000 (UTC) Subject: Re: [PATCH v2 1/4] mm: Fix multiple evaluvations of totalram_pages and managed_pages To: Michal Hocko , Arun KS Cc: akpm@linux-foundation.org, keescook@chromium.org, khlebnikov@yandex-team.ru, minchan@kernel.org, osalvador@suse.de, linux-kernel@vger.kernel.org, linux-mm@kvack.org, getarunks@gmail.com References: <1541521310-28739-1-git-send-email-arunks@codeaurora.org> <1541521310-28739-2-git-send-email-arunks@codeaurora.org> <20181107082037.GX27423@dhcp22.suse.cz> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; prefer-encrypt=mutual; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20181107082037.GX27423@dhcp22.suse.cz> 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 9:20 AM, Michal Hocko wrote: > On Tue 06-11-18 21:51:47, Arun KS wrote: Hi, there's typo in subject: evaluvations -> evaluations. However, "fix" is also misleading (more below), so I'd suggest something like: mm: reference totalram_pages and managed_pages once per function >> This patch is in preparation to a later patch which converts totalram_pages >> and zone->managed_pages to atomic variables. This patch does not introduce >> any functional changes. > > I forgot to comment on this one. The patch makes a lot of sense. But I > would be little bit more conservative and won't claim "no functional > changes". As things stand now multiple reads in the same function are > racy (without holding the lock). I do not see any example of an > obviously harmful case but claiming the above is too strong of a > statement. I would simply go with something like "Please note that > re-reading the value might lead to a different value and as such it > could lead to unexpected behavior. There are no known bugs as a result > of the current code but it is better to prevent from them in principle." However, the new code doesn't use READ_ONCE(), so the compiler is free to read the value multiple times, and before the patch it was free to read it just once, as the variables are not volatile. So strictly speaking this is indeed not a functional change (if compiler decides differently based on the patch, it's an implementation detail). So even in my suggested subject above, 'reference' is meant as a source code reference, not really a memory read reference. Couldn't think of a better word though.