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 351DBC433E4 for ; Wed, 24 Mar 2021 19:18:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 127B561A17 for ; Wed, 24 Mar 2021 19:18:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237855AbhCXTSa (ORCPT ); Wed, 24 Mar 2021 15:18:30 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:51919 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237943AbhCXTSH (ORCPT ); Wed, 24 Mar 2021 15:18:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1616613486; 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=L6QfMU6ICmnSA0e3EzDjVM0dhhad75vCN7chXuQhF9w=; b=UOozatgP5mYyV6Pa2qrrJM5L2oomrPhromtKR3XwXpXhuqZDa7e9WA9H6fgc3q/HiaXcLP si1o7rVsVgHFTY19uE6dR6W/dUUMS1iNOfqPuNOalbF6EfX+qdugxpWZorpzhk3lEoc77T gWFWFq3NTN9Dda+2DqCvFdNZvAAerLk= 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-28-rNCPKapKMgqOwm393TehQA-1; Wed, 24 Mar 2021 15:18:02 -0400 X-MC-Unique: rNCPKapKMgqOwm393TehQA-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5CCD21020357; Wed, 24 Mar 2021 19:16:57 +0000 (UTC) Received: from [10.36.115.66] (ovpn-115-66.ams2.redhat.com [10.36.115.66]) by smtp.corp.redhat.com (Postfix) with ESMTP id DABA25D9CA; Wed, 24 Mar 2021 19:16:54 +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: <20210319092635.6214-1-osalvador@suse.de> <20210319092635.6214-2-osalvador@suse.de> <20210324101259.GB16560@linux> <3bc4168c-fd31-0c9a-44ac-88e25d524eef@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: <9591a0b8-c000-2f61-67a6-4402678fe50b@redhat.com> Date: Wed, 24 Mar 2021 20:16:53 +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 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 24.03.21 17:04, Michal Hocko wrote: > On Wed 24-03-21 15:52:38, David Hildenbrand wrote: >> On 24.03.21 15:42, Michal Hocko wrote: >>> On Wed 24-03-21 13:03:29, Michal Hocko wrote: >>>> On Wed 24-03-21 11:12:59, Oscar Salvador wrote: >>> [...] >>>>> I kind of understand to be reluctant to use vmemmap_pages terminology here, but >>>>> unfortunately we need to know about it. >>>>> We could rename nr_vmemmap_pages to offset_buddy_pages or something like that. >>>> >>>> I am not convinced. It seems you are justr trying to graft the new >>>> functionality in. But I still believe that {on,off}lining shouldn't care >>>> about where their vmemmaps come from at all. It should be a >>>> responsibility of the code which reserves that space to compansate for >>>> accounting. Otherwise we will end up with a hard to maintain code >>>> because expectations would be spread at way too many places. Not to >>>> mention different pfns that the code should care about. >>> >>> The below is a quick hack on top of this patch to illustrate my >>> thinking. I have dug out all the vmemmap pieces out of the >>> {on,off}lining and hooked all the accounting when the space is reserved. >>> This just compiles without any deeper look so there are likely some >>> minor problems but I haven't really encountered any major problems or >>> hacks to introduce into the code. The separation seems to be possible. >>> The diffstat also looks promising. Am I missing something fundamental in >>> this? >>> >> >> From a quick glimpse, this touches on two things discussed in the past: >> >> 1. If the underlying memory block is offline, all sections are offline. Zone >> shrinking code will happily skip over the vmemmap pages and you can end up >> with out-of-zone pages assigned to the zone. Can happen in corner cases. > > You are right. But do we really care? Those pages should be of no > interest to anybody iterating through zones/nodes anyway. Well, we were just discussing getting zone/node links + span right for all pages (including for special reserved pages), because it already resulted in BUGs. So I am not convinced that we *don't* have to care. However, I agree that most code that cares about node/zone spans shouldn't care - e.g., never call set_pfnblock_flags_mask() on such blocks. But I guess there are corner cases where we would end up with zone_is_empty() == true, not sure what that effect would be ... at least the node cannot vanish as we disallow offlining it while we have a memory block linked to it. Another thing that comes to my mind is that our zone shrinking code currently searches in PAGES_PER_SUBSECTION (2 MiB IIRC) increments. In case our vmemmap pages would be less than that, we could accidentally shrink the !vmemmap part too much, as we are mis-detecting the type for a PAGES_PER_SUBSECTION block. IIRC, this would apply for memory block sizes < 128 MiB. Not relevant on x86 and arm64. Could be relevant for ppc64, if we'd ever want to support memmap_on_memory there. Or if we'd ever reduce the section size on some arch below 128 MiB. At least we would have to fence it somehow. > >> There is no way to know that the memmap of these pages was initialized and >> is of value. >> >> 2. You heavily fragment zone layout although you might end up with >> consecutive zones (e.g., online all hotplugged memory movable) > > What would be consequences? IIRC, set_zone_contiguous() will leave zone->contiguous = false. This, in turn, will force pageblock_pfn_to_page() via the slow path, turning page isolation a bit slower. Not a deal breaker, but obviously something where Oscar's original patch can do better. I yet have to think again about other issues (I remember most issues we discussed back then were related to having the vmemmap only within the same memory block). I think 2) might be tolerable, although unfortunate. Regarding 1), we'll have to dive into more details. -- Thanks, David / dhildenb