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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BD106C77B72 for ; Mon, 17 Apr 2023 11:29:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230445AbjDQL3c (ORCPT ); Mon, 17 Apr 2023 07:29:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51052 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230171AbjDQL3a (ORCPT ); Mon, 17 Apr 2023 07:29:30 -0400 Received: from outbound-smtp51.blacknight.com (outbound-smtp51.blacknight.com [46.22.136.235]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1D1E4272A for ; Mon, 17 Apr 2023 04:28:36 -0700 (PDT) Received: from mail.blacknight.com (pemlinmail04.blacknight.ie [81.17.254.17]) by outbound-smtp51.blacknight.com (Postfix) with ESMTPS id 2B1A6FA943 for ; Mon, 17 Apr 2023 12:13:54 +0100 (IST) Received: (qmail 19061 invoked from network); 17 Apr 2023 11:13:54 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.21.103]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 17 Apr 2023 11:13:54 -0000 Date: Mon, 17 Apr 2023 12:13:52 +0100 From: Mel Gorman To: Wen Yang Cc: Andrew Morton , Oscar Salvador , William Lam , Fu Wei , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: compaction: optimize compact_memory to comply with the admin-guide Message-ID: <20230417111352.v26slrcmz4qo3tnn@techsingularity.net> References: <20230411134801.a4aadef5aba0f51e0d44bb7a@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Apr 16, 2023 at 01:42:44AM +0800, Wen Yang wrote: > > ??? 2023/4/13 00:54, Wen Yang ??????: > > > > ??? 2023/4/12 04:48, Andrew Morton ??????: > > > On Wed, 12 Apr 2023 02:24:26 +0800 wenyang.linux@foxmail.com wrote: > > > > > > > For the /proc/sys/vm/compact_memory file, the admin-guide states: > > > > When 1 is written to the file, all zones are compacted such that free > > > > memory is available in contiguous blocks where possible. This can be > > > > important for example in the allocation of huge pages although > > > > processes > > > > will also directly compact memory as required > > > > > > > > But it was not strictly followed, writing any value would cause all > > > > zones to be compacted. In some critical scenarios, some applications > > > > operating it, such as echo 0, have caused serious problems. > > > Really?  You mean someone actually did this and didn't observe the > > > effect during their testing? > > > > Thanks for your reply. > > > > Since /proc/sys/vm/compact_memory has been well documented for over a > > decade: > > > > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Documentation/admin-guide/sysctl/vm.rst#n109 > > > > > > it is believed that only writing 1 will trigger trigger all zones to be > > compacted. > > > > Especially for those who write applications, they may only focus on > > documentation and generally do not read kernel code.  Moreover, such > > problems are not easily detected through testing on low pressure > > machines. > > > > Writing any meaningful or meaningless values will trigger it and affect > > the entire server: > > > > # echo 1 > /proc/sys/vm/compact_memory > > # echo 0 > /proc/sys/vm/compact_memory > > # echo dead > /proc/sys/vm/compact_memory > > # echo "hello world" > /proc/sys/vm/compact_memory > > > > The implementation of this high-risk operation may require following the > > admin-guides. > > > > -- > > > > Best wishes, > > > > Wen > > > > > Hello, do you think it's better to optimize the sysctl_compaction_handler > code or update the admin-guide document? > Enforce the 1 on the unlikely chance that the sysctl handler is ever extended to do something different and expects a bitmask. The original intent intent of the sysctl was debugging -- demonstrating a contiguous allocation failure when aggressive compaction should have succeeded. Later some machines dedicated to batch jobs used the compaction sysctl to compact memory before a new job started to reduce startup latencies. Drop the justification "In some critical scenarios, some applications operating it, such as echo 0, have caused serious problems." from the changelog. I cannot imagine a sane "critical scenario" where an application running as root is writing expected garbage to proc or sysfs files and then surprised when something unexpected happens. -- Mel Gorman SUSE Labs