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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 3DA24C5ACC5 for ; Wed, 19 Feb 2020 23:08:08 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 053182464E for ; Wed, 19 Feb 2020 23:08:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 053182464E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 96B266B0005; Wed, 19 Feb 2020 18:08:07 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 91C036B0006; Wed, 19 Feb 2020 18:08:07 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 80AA86B0007; Wed, 19 Feb 2020 18:08:07 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0068.hostedemail.com [216.40.44.68]) by kanga.kvack.org (Postfix) with ESMTP id 6A28B6B0005 for ; Wed, 19 Feb 2020 18:08:07 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 260D98248D51 for ; Wed, 19 Feb 2020 23:08:07 +0000 (UTC) X-FDA: 76508416614.30.table69_63ef33c612c06 X-HE-Tag: table69_63ef33c612c06 X-Filterd-Recvd-Size: 3312 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf08.hostedemail.com (Postfix) with ESMTP for ; Wed, 19 Feb 2020 23:08:06 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AEC5A1FB; Wed, 19 Feb 2020 15:08:05 -0800 (PST) Received: from [192.168.1.123] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4F30D3F703; Wed, 19 Feb 2020 15:08:03 -0800 (PST) Subject: Re: [RFC PATCH] memory_hotplug: disable the functionality for 32b To: Andrew Morton , Michal Hocko Cc: linux-arch@vger.kernel.org, bhe@redhat.com, david@redhat.com, bugzilla-daemon@bugzilla.kernel.org, linux-mm@kvack.org, richardw.yang@linux.intel.com, n-horiguchi@ah.jp.nec.com, kkabe@vega.pgw.jp, linux-arm-kernel@lists.infradead.org, osalvador@suse.de References: <20200218084700.GD21113@dhcp22.suse.cz> <200218181900.M0115079@vega.pgw.jp> <20200218100532.GA4151@dhcp22.suse.cz> <20200219134645.7430db57e0e59f69e7386f46@linux-foundation.org> From: Robin Murphy Message-ID: <8877ad4c-00c0-0e7a-5515-533d85014bdd@arm.com> Date: Wed, 19 Feb 2020 23:07:54 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:68.0) Gecko/20100101 Thunderbird/68.4.2 MIME-Version: 1.0 In-Reply-To: <20200219134645.7430db57e0e59f69e7386f46@linux-foundation.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2020-02-19 9:46 pm, Andrew Morton wrote: > On Tue, 18 Feb 2020 11:05:32 +0100 Michal Hocko wrote: > >> Subject: [PATCH] memory_hotplug: disable the functionality for 32b >> >> Memory hotlug is broken for 32b systems at least since c6f03e2903c9 >> ("mm, memory_hotplug: remove zone restrictions") which has considerably >> reworked how can be memory associated with movable/kernel zones. The >> same is not really trivial to achieve in 32b where only lowmem is the >> kernel zone. While we can tweak this immediate problem around there are >> likely other land mines hidden at other places. >> >> It is also quite dubious that there is a real usecase for the memory >> hotplug on 32b in the first place. Low memory is just too small to be >> hotplugable (for hot add) and generally unusable for hotremove. Adding >> more memory to highmem is also dubious because it would increase the >> low mem or vmalloc space pressure for memmaps. >> >> Restrict the functionality to 64b systems. This will help future >> development to focus on usecases that have real life application. We >> can remove this restriction in future in presence of a real life usecase >> of course but until then make it explicit that hotplug on 32b is broken >> and requires a non trivial amount of work to fix. > > (cc linux-arch) > > (and linux-arm-kernel, as ARM is a major 32-bit user) > > Does anyone see a problem with disabling memory hotplug on 32-bit builds? 32-bit Arm doesn't support memory hotplug, and as far as I'm aware there's little likelihood of it ever wanting to. FWIW it looks like SuperH is the only pure-32-bit architecture to have hotplug support at all. Robin.