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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 0C54CC433DB for ; Thu, 25 Mar 2021 10:56:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CD18D619B4 for ; Thu, 25 Mar 2021 10:56:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229631AbhCYKzb (ORCPT ); Thu, 25 Mar 2021 06:55:31 -0400 Received: from mx2.suse.de ([195.135.220.15]:41480 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230317AbhCYKzF (ORCPT ); Thu, 25 Mar 2021 06:55:05 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 333A8ADD7; Thu, 25 Mar 2021 10:55:04 +0000 (UTC) Date: Thu, 25 Mar 2021 11:55:01 +0100 From: Oscar Salvador To: Michal Hocko Cc: David Hildenbrand , Andrew Morton , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Message-ID: References: <20210319092635.6214-2-osalvador@suse.de> <20210324101259.GB16560@linux> <3bc4168c-fd31-0c9a-44ac-88e25d524eef@redhat.com> <9591a0b8-c000-2f61-67a6-4402678fe50b@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 25, 2021 at 10:17:33AM +0100, Michal Hocko wrote: > Why do you think it is wrong to initialize/account pages when they are > used? Keep in mind that offline pages are not used until they are > onlined. But vmemmap pages are used since the vmemmap is established > which happens in the hotadd stage. Yes, that is true. vmemmap pages are used right when we populate the vmemmap space. > > plus the fact that I dislike to place those pages in > > ZONE_NORMAL, although they are not movable. > > But I think the vmemmap pages should lay within the same zone the pages > > they describe, doing so simplifies things, and I do not see any outright > > downside. > > Well, both ways likely have its pros and cons. Nevertheless, if the > vmemmap storage is independent (which is the case for normal hotplug) > then the state is consistent over hotadd, {online, offline} N times, > hotremove cycles. Which is conceptually reasonable as vmemmap doesn't > go away on each offline. > > If you are going to bind accounting to the online/offline stages then > the accounting changes each time you go through the cycle and depending > on the onlining type it would travel among zones. I find it quite > confusing as the storage for vmemmap hasn't changed any of its > properties. That is a good point I guess. vmemmap pages do not really go away until the memory is unplugged. But I see some questions to raise: - As I said, I really dislike it tiding vmemmap memory to ZONE_NORMAL unconditionally and this might result in the problems David mentioned. I remember David and I discussed such problems but the problems with zones not being contiguos have also been discussed in the past and IIRC, we reached the conclusion that a maximal effort should be made to keep them that way, otherwise other things suffer e.g: compaction code. So if we really want to move the initialization/account to the hot-add/hot-remove stage, I would really like to be able to set the proper zone in there (that is, the same zone where the memory will lay). - When moving the initialization/accounting to hot-add/hot-remove, the section containing the vmemmap pages will remain offline. It might get onlined once the pages get online in online_pages(), or not if vmemmap pages span a whole section. I remember (but maybe David rmemeber better) that that was a problem wrt. pfn_to_online_page() and hybernation/kdump. So, if that is really a problem, we would have to care of ot setting the section to the right state. - AFAICS, doing all the above brings us to former times were some initialization/accounting was done in a previous stage, and I remember it was pushed hard to move those in online/offline_pages(). Are we ok with that? As I said, we might have to set the right zone in hot-add stage, as otherwise problems might come up. Being that case, would not that also be conflating different concepts at a wrong phases? Do not take me wrong, I quite like Michal's idea, and from a conceptually point of view I guess it is the right thing to do. But when evualating risks/difficulty, I am not really sure. If we can pull that off while setting the right zone (and must be seen what about the section state), and the outcome is not ugly, I am all for it. Also a middel-ground might be something like I previously mentioned(having a helper in memory_block_action() to do the right thing, so offline/online_pages() do not get pouled. -- Oscar Salvador SUSE L3