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=-7.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 66729C433E3 for ; Thu, 25 Mar 2021 12:41:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 392C461A38 for ; Thu, 25 Mar 2021 12:41:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231258AbhCYMlC (ORCPT ); Thu, 25 Mar 2021 08:41:02 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:28200 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230239AbhCYMkz (ORCPT ); Thu, 25 Mar 2021 08:40:55 -0400 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 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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. >>> >> >> Note: I once herd of a corner-case use case where people offline memory >> 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 ... > > 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 hotadded > is not useable. I think they can use it just fine via /dev/mem, which explicitly avoids any kind of "struct page" references IIRC. They would be overwriting the vmemmap, but that part scan happily read/write until onlining, where the vmemmap would get reinitialized and set online - from which point on pfn_to_online_page() would succeed also on the vmemmap itself. > > [...] >>> - 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. >> >> Good memory. Indeed, hibernation/kdump won't save the state of the vmemmap, >> because the memory is marked as offline and, thus, logically without any >> valuable content. > > 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(). > > 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 about memory blocks having their altmap located on other memory blocks. > > >>> - 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? >>> >> >> 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. > > 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 :) -- Thanks, David / dhildenb