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 44703C433E9 for ; Thu, 18 Mar 2021 11:25:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EBA5F64E61 for ; Thu, 18 Mar 2021 11:25:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230014AbhCRLYy (ORCPT ); Thu, 18 Mar 2021 07:24:54 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:43732 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229803AbhCRLYX (ORCPT ); Thu, 18 Mar 2021 07:24:23 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1616066663; 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=CKgKig4qhgfYAN5+Xyhcsw2h6SqxVFiqlkAsYQ9Uxzk=; b=VORpiYkA/QsPCekkF8VTA0z1QR/17TlQ/5FwwA0bPFkXNy00JxfeVEBPAeYy5qryxLcKc+ euApSAwe55ay3rUz1SogfFfUjfa+ufPr7lAlRWcSWAY38jE8DQE5oNUKlui3BfTEd1A2Lg cery1gVxjcWiJJq3sD1xnLSdxCybxWo= 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-341-7p8QkfV3OtC-7pYIeYaAnw-1; Thu, 18 Mar 2021 07:24:21 -0400 X-MC-Unique: 7p8QkfV3OtC-7pYIeYaAnw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 74AE88189C6; Thu, 18 Mar 2021 11:24:19 +0000 (UTC) Received: from [10.36.113.61] (ovpn-113-61.ams2.redhat.com [10.36.113.61]) by smtp.corp.redhat.com (Postfix) with ESMTP id 696DE5C1D1; Thu, 18 Mar 2021 11:24:17 +0000 (UTC) To: Oscar Salvador Cc: Andrew Morton , Michal Hocko , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20210309175546.5877-1-osalvador@suse.de> <20210309175546.5877-2-osalvador@suse.de> <20210315102224.GA24699@linux> <20210317140847.GA20407@linux> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v4 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Message-ID: <51c645b3-1220-80c4-e44c-4c0411222148@redhat.com> Date: Thu, 18 Mar 2021 12:24:16 +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 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18.03.21 11:38, Oscar Salvador wrote: > On Thu, Mar 18, 2021 at 09:27:48AM +0100, Oscar Salvador wrote: >>> If we check for >>> >>> IS_ALIGNED(nr_vmemmap_pages, PMD_SIZE), please add a proper TODO comment >>> that this is most probably the wrong place to take care of this. >> >> Sure, I will stuff the check in there and place a big TODO comment so we >> do not forget about addressing this issue the right way. > > Ok, I realized something while working on v5. > > Here is what I have right now: > > bool mhp_supports_memmap_on_memory(unsigned long size) > { > /* > * Note: We calculate for a single memory section. The calculation > * implicitly covers memory blocks that span multiple sections. > * > * Not all archs define SECTION_SIZE, but MIN_MEMORY_BLOCK_SIZE always > * equals SECTION_SIZE, so use that instead. > */ > unsigned long nr_vmemmap_pages = MIN_MEMORY_BLOCK_SIZE / PAGE_SIZE; Even clearer would be just using "size / PAGE_SIZE" here. The you can even drop the comment. > unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page); > unsigned long remaining_size = size - vmemmap_size; > > /* > * Besides having arch support and the feature enabled at runtime, we > * need a few more assumptions to hold true: > * > * a) We span a single memory block: memory onlining/offlinin;g happens > * in memory block granularity. We don't want the vmemmap of online > * memory blocks to reside on offline memory blocks. In the future, > * we might want to support variable-sized memory blocks to make the > * feature more versatile. > * > * b) The vmemmap pages span complete PMDs: We don't want vmemmap code > * to populate memory from the altmap for unrelated parts (i.e., > * other memory blocks) > * > * c) The vmemmap pages (and thereby the pages that will be exposed to > * the buddy) have to cover full pageblocks: memory onlining/offlining > * code requires applicable ranges to be page-aligned, for example, to > * set the migratetypes properly. > * > * TODO: Although we have a check here to make sure that vmemmap pages > * fully populate a PMD, it is not the right place to check for > * this. A much better solution involves improving vmemmap code > * to fallback to base pages when trying to populate vmemmap using > * altmap as an alternative source of memory, and we do not exactly > * populate a single PMD. > */ > return memmap_on_memory && > IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) && > size == memory_block_size_bytes() && > remaining_size && > IS_ALIGNED(remaining_size, pageblock_size) && > IS_ALIGNED(vmemmap_size, PMD_SIZE); > } > > Assume we are on x86_64 to simplify the case. > > Above, nr_vmemmap_pages would be 32768 and vmemmap_size 2MB (exactly a > PMD). > > Now, although correct, this nr_vmemmap_pages does not match with the > altmap->alloc. > > static void * __meminit altmap_alloc_block_buf(unsigned long size, > struct altmap) > { > ... > ... > nr_pfns = size >> PAGE_SHIFT; //size is PMD_SIZE > altmap->alloc += nr_pfns; > } > > altmap->alloc will be 512, 512 * 4K pages = 2MB. > > Of course, the reason they do not match is because in one case, we are > saying a) how many pfns we need to cover a PMD_SIZE, while in the > other case we say b) how many pages we need to cover SECTION_SIZE > > Then b) multiply for page_size to get the current size of it. I don't follow. 2MB == 2MB. And if there would be difference then we would be in the problem I brought up: vmemmap code allocating too much via the altmap, which can be very bad because might be populating more vmemmap than we actually need. > > So, I have mixed feeling about this. > Would it be more clear to just do: > > bool mhp_supports_memmap_on_memory(unsigned long size) > { > /* > * Note: We calculate for a single memory section. The calculation > * implicitly covers memory blocks that span multiple sections. > */ Then this comment is wrong > unsigned long nr_vmemmap_pages = PMD_SIZE / PAGE_SIZE; And this stuff just gets confusing. nr_vmemmap_pages = 2MiB / 4 KiB = 512; > unsigned long vmemmap_size = nr_vmemmap_pages * PAGE_SIZE; vmemmap_size = 512 * 4KiB = 2 MiB. That calculation wasn't very useful (/ PAGE_SIZE * PAGE_SIZE)? > unsigned long remaining_size = size - vmemmap_size; And here we could get something like remaining_size = 2 GiB - 2 MiB ? Which does not make any sense. > ... > ... > > -- Thanks, David / dhildenb