From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from outbound-smtp32.blacknight.com (outbound-smtp32.blacknight.com [81.17.249.64]) by mx.groups.io with SMTP id smtpd.web10.17536.1601839483336769238 for ; Sun, 04 Oct 2020 12:24:44 -0700 Authentication-Results: mx.groups.io; dkim=missing; spf=pass (domain: techsingularity.net, ip: 81.17.249.64, mailfrom: mgorman@techsingularity.net) Received: from mail.blacknight.com (pemlinmail03.blacknight.ie [81.17.254.16]) by outbound-smtp32.blacknight.com (Postfix) with ESMTPS id 88BC1BEEC7 for ; Sun, 4 Oct 2020 20:24:40 +0100 (IST) Received: (qmail 16067 invoked from network); 4 Oct 2020 19:24:40 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.22.4]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 4 Oct 2020 19:24:40 -0000 Date: Sun, 4 Oct 2020 20:24:37 +0100 From: Mel Gorman To: Lukas Bulwahn Cc: Andrew Morton , linux-mm@kvack.org, Vlastimil Babka , Michal Hocko , Nathan Chancellor , Nick Desaulniers , linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com, kernel-janitors@vger.kernel.org, linux-safety@lists.elisa.tech Subject: Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd() Message-ID: <20201004192437.GF3227@techsingularity.net> References: <20201004125827.17679-1-lukas.bulwahn@gmail.com> MIME-Version: 1.0 In-Reply-To: <20201004125827.17679-1-lukas.bulwahn@gmail.com> User-Agent: Mutt/1.10.1 (2018-07-13) Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline On Sun, Oct 04, 2020 at 02:58:27PM +0200, Lukas Bulwahn wrote: > The refactoring to kswapd() in commit e716f2eb24de ("mm, vmscan: prevent > kswapd sleeping prematurely due to mismatched classzone_idx") turned an > assignment to reclaim_order into a dead store, as in all further paths, > reclaim_order will be assigned again before it is used. > > make clang-analyzer on x86_64 tinyconfig caught my attention with: > > mm/vmscan.c: warning: Although the value stored to 'reclaim_order' is > used in the enclosing expression, the value is never actually read from > 'reclaim_order' [clang-analyzer-deadcode.DeadStores] > > Compilers will detect this unneeded assignment and optimize this anyway. > So, the resulting binary is identical before and after this change. > > Simplify the code and remove unneeded assignment to make clang-analyzer > happy. > > No functional change. No change in binary code. > > Signed-off-by: Lukas Bulwahn I'm not really keen on this. With the patch, reclaim_order can be passed uninitialised to kswapd_try_to_sleep. While a sufficiently smart compiler might be able to optimise how reclaim_order is used, it's not guaranteed either. Similarly, a change in kswapd_try_to_sleep and its called functions could rely on reclaim_order being a valid value and then introduce a subtle bug. -- Mel Gorman SUSE Labs