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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, 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 6100EC433E2 for ; Thu, 25 Mar 2021 12:40:57 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id E0D5761A25 for ; Thu, 25 Mar 2021 12:40:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E0D5761A25 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 7B7AE6B0070; Thu, 25 Mar 2021 08:40:56 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7678E6B0071; Thu, 25 Mar 2021 08:40:56 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5E18E6B0072; Thu, 25 Mar 2021 08:40:56 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0051.hostedemail.com [216.40.44.51]) by kanga.kvack.org (Postfix) with ESMTP id 3E29D6B0070 for ; Thu, 25 Mar 2021 08:40:56 -0400 (EDT) Received: from smtpin17.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id F054318403C14 for ; Thu, 25 Mar 2021 12:40:55 +0000 (UTC) X-FDA: 77958356070.17.E06D233 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by imf04.hostedemail.com (Postfix) with ESMTP id D4C733C0 for ; Thu, 25 Mar 2021 12:40:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1616676054; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=YOKQNCijii7AvVDUdkRjAl0wBlJEDwK56PtHxBH068U=; b=Mx22iVg5S9fgnUMQsHQJO6wc8cOvHT9eK4IsRfEb9plQr8Hp9Od7ebOqQsILvcsORavy0X cOwDwHazpagaeDPssJ6vouS2ooaDwOEXRId/+Fays5feZC09JX8VF8pfkZJzTopdzXzJKa sJX9/5gLRTuEn5IQU6M4tbfxgxzjyf0= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-599-vhZ3or8tPvKgo6MfvPI6dA-1; Thu, 25 Mar 2021 08:40:50 -0400 X-MC-Unique: vhZ3or8tPvKgo6MfvPI6dA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 1984D814256; Thu, 25 Mar 2021 12:40:49 +0000 (UTC) Received: from [10.36.115.72] (ovpn-115-72.ams2.redhat.com [10.36.115.72]) by smtp.corp.redhat.com (Postfix) with ESMTP id CB46E60CCF; Thu, 25 Mar 2021 12:40:46 +0000 (UTC) To: Michal Hocko Cc: Oscar Salvador , Andrew Morton , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20210324101259.GB16560@linux> <3bc4168c-fd31-0c9a-44ac-88e25d524eef@redhat.com> <9591a0b8-c000-2f61-67a6-4402678fe50b@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Message-ID: <31110e58-c99a-8dee-6f6e-98f456b77759@redhat.com> Date: Thu, 25 Mar 2021 13:40:45 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Stat-Signature: f7qoo7pequ5u3jx79mekj9uarqfoy6pn X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: D4C733C0 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf04; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616676053-897350 Content-Transfer-Encoding: quoted-printable 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 25.03.21 13:35, Michal Hocko wrote: > On Thu 25-03-21 12:08:43, David Hildenbrand wrote: >> On 25.03.21 11:55, Oscar Salvador wrote: >>> 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 a= re >>>> 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. >>> >> >> Note: I once herd of a corner-case use case where people offline memor= y >> blocks to then use the "free" memory via /dev/mem for other purposes (= "large >> physical memory"). Not that I encourage such use cases, but they would= be >> fundamentally broken if the vmemmap ends up on offline memory and is >> supposed to keep its state ... >=20 > I am not aware of such a use case, it surely sounds, ehm creative, but > nothing really new. But such a usecase sounds quite incompatible with > this feature whether we account vmemmap at hotadd or hotremove because > they would need to understand that part of the memory they have hotadde= d > is not useable. I think they can use it just fine via /dev/mem, which explicitly avoids=20 any kind of "struct page" references IIRC. They would be overwriting the=20 vmemmap, but that part scan happily read/write until onlining, where the=20 vmemmap would get reinitialized and set online - from which point on=20 pfn_to_online_page() would succeed also on the vmemmap itself. >=20 > [...] >>> - 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 prob= lem >>> wrt. pfn_to_online_page() and hybernation/kdump. >>> So, if that is really a problem, we would have to care of ot sett= ing >>> the section to the right state. >> >> Good memory. Indeed, hibernation/kdump won't save the state of the vme= mmap, >> because the memory is marked as offline and, thus, logically without a= ny >> valuable content. >=20 > Could you point me to the respective hibernation code please? I always > get lost in that area. Anyway, we do have the same problem even if the > whole accounting is handled during {on,off}lining, no? kernel/power/snapshot.c:saveable_page(). >=20 > I am not really worried about kdump though. As the memory is offline > then losing itse vmemmap is a mere annoyance. Yep, kdump was relevant in our previous discussions when we were talking=20 about memory blocks having their altmap located on other memory blocks. >=20 >=20 >>> - AFAICS, doing all the above brings us to former times were some >>> initialization/accounting was done in a previous stage, and I rem= ember >>> 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 conc= epts >>> at a wrong phases? >>> >> >> I expressed my opinion already, no need to repeat. Sub-section online = maps >> would make it cleaner, but I am still not convinced we want/need that. >=20 > Nah, subsections are more tricky than necessary and if we can live > without them and have it just as pmem weirdness is more than enough ;) Yes, absolutely :) --=20 Thanks, David / dhildenb