From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from outbound-smtp09.blacknight.com (outbound-smtp09.blacknight.com [46.22.139.14]) by mx.groups.io with SMTP id smtpd.web11.29735.1601884750227858749 for ; Mon, 05 Oct 2020 00:59:10 -0700 Authentication-Results: mx.groups.io; dkim=missing; spf=pass (domain: techsingularity.net, ip: 46.22.139.14, mailfrom: mgorman@techsingularity.net) Received: from mail.blacknight.com (pemlinmail02.blacknight.ie [81.17.254.11]) by outbound-smtp09.blacknight.com (Postfix) with ESMTPS id 75BE21C5115 for ; Mon, 5 Oct 2020 08:59:06 +0100 (IST) Received: (qmail 27315 invoked from network); 5 Oct 2020 07:59:06 -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); 5 Oct 2020 07:59:06 -0000 Date: Mon, 5 Oct 2020 08:59:03 +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: <20201005075903.GH3227@techsingularity.net> References: <20201004125827.17679-1-lukas.bulwahn@gmail.com> <20201004192437.GF3227@techsingularity.net> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline On Mon, Oct 05, 2020 at 08:58:53AM +0200, Lukas Bulwahn wrote: > > > On Sun, 4 Oct 2020, Mel Gorman wrote: > > > 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. > > > > Just for my own understanding: > > How would you see reclaim_order being passed unitialised to > kswapd_try_to_sleep? > > From kswapd() entry, any path must reach the line > > alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order); > > before kswap_try_to_sleep(...). > Bah, I misread the patch because I'm an idiot. Acked-by: Mel Gorman -- Mel Gorman SUSE Labs