From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) by kanga.kvack.org (Postfix) with ESMTP id 94D976B04D5 for ; Wed, 7 Nov 2018 04:23:57 -0500 (EST) Received: by mail-ed1-f69.google.com with SMTP id y72-v6so9105720ede.22 for ; Wed, 07 Nov 2018 01:23:57 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id o1-v6si250832edf.177.2018.11.07.01.23.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 07 Nov 2018 01:23:56 -0800 (PST) Date: Wed, 7 Nov 2018 10:23:54 +0100 From: Michal Hocko Subject: Re: [PATCH v2 1/4] mm: Fix multiple evaluvations of totalram_pages and managed_pages Message-ID: <20181107092354.GZ27423@dhcp22.suse.cz> References: <1541521310-28739-1-git-send-email-arunks@codeaurora.org> <1541521310-28739-2-git-send-email-arunks@codeaurora.org> <20181107082037.GX27423@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Arun KS , 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 On Wed 07-11-18 09:44:00, Vlastimil Babka wrote: > 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). Yes, compiler is allowed to optimize this either way without READ_ONCE but it is allowed to do two reads so claiming no functional change is a bit problematic. Not that this would be a reason to discuss this in length... -- Michal Hocko SUSE Labs