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=-7.7 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 463B3C433DB for ; Fri, 8 Jan 2021 15:32:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E453E23A1E for ; Fri, 8 Jan 2021 15:32:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727824AbhAHPcG (ORCPT ); Fri, 8 Jan 2021 10:32:06 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:29461 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725806AbhAHPcF (ORCPT ); Fri, 8 Jan 2021 10:32:05 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1610119839; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=sWxbH/EEwDRQhL/kJPvMQX0Ji74HwvfBlAxAy5TRY6E=; b=DnLJVPrcZ7T+2BOybQ13cSNqSYrCeT45OwjT9+68x0pWRqHC/jn2qUo5fmSe1xN0fu1Aey /BltyCFlW6mYf1G30hUJgTmMf1tsvDbLNfcu/XBCsHtUomQrCW6mOko2GBg46xhu7Awri8 UEvURxc0CNC2prXvy41i26yWCLuLQNQ= 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-194-mjoiOGY4PHavf52dOOIAJg-1; Fri, 08 Jan 2021 10:30:35 -0500 X-MC-Unique: mjoiOGY4PHavf52dOOIAJg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id AF3F9809DCE; Fri, 8 Jan 2021 15:30:33 +0000 (UTC) Received: from [10.36.114.168] (ovpn-114-168.ams2.redhat.com [10.36.114.168]) by smtp.corp.redhat.com (Postfix) with ESMTP id 64A4B12D7E; Fri, 8 Jan 2021 15:30:32 +0000 (UTC) Subject: Re: [PATCH 1/1] arm64: make section size configurable for memory hotplug To: Anshuman Khandual , Sudarshan Rajagopalan , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <66f79b0c06602c22df4da8ff4a5c2b97c9275250.1609895500.git.sudaraja@codeaurora.org> <055b0aca-af60-12ad-cd68-e15440ade64b@arm.com> <3ae8c16d-50c4-c6cc-62b8-922cfc308c95@arm.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <7939710a-5d03-de2b-73b2-bca472de431a@redhat.com> Date: Fri, 8 Jan 2021 16:30:31 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <3ae8c16d-50c4-c6cc-62b8-922cfc308c95@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > To summarize, the section size bits for each base page size config > should always > > a. Avoid (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS Pageblocks must also always fall completely into a section. > > b. Provide minimum possible section size for a given base page config to > have increased agility during memory hotplug operations and reduced > vmemmap wastage for sections with holes. OTOH, making the section size too small (e.g., 16MB) creates way to many memory block devices in /sys/devices/system/memory/, and might consume too many page->flags bits in the !vmemmap case. For bigger setups, we might, similar to x86-64 (e.g., >= 64 GiB), determine the memory_block_size_bytes() at runtime (spanning multiple sections then), once it becomes relevant. > > c. Allow 4K base page configs to have PMD based vmemmap mappings Agreed. > > Because CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64 platform, > the following would always avoid the condition (a) > > SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) > > - 22 (11 - 1 + 12) for 4K pages > - 24 (11 - 1 + 14) for 16K pages without THP > - 25 (12 - 1 + 14) for 16K pages with THP > - 26 (11 - 1 + 16) for 64K pages without THP > - 29 (14 - 1 + 16) for 64K pages with THP > > Apart from overriding 4K base page size config to have 27 as section size > bits, should not all other values be okay here ? But then wondering what > benefit 128MB (27 bits) section size for 16K config would have ? OR the > objective here is to match 16K page size config with default x86-64. We don't want to have sections that are too small. We don't want to have sections that are too big :) Not sure if we really want to allow setting e.g., a section size of 4 MB. That's just going to hurt. IMHO, something in the range of 64..256 MB is quite a good choice, where possible. > >> >> (If we worry about the number of section bits in page->flags, we could >> glue it to vmemmap support where that does not matter) > > Could you please elaborate ? Could smaller section size bits numbers like > 22 or 24 create problems in page->flags without changing other parameters > like NR_CPUS or NODES_SHIFT ? A quick test with 64K base page without THP Yes, in the !vmemmap case, we have to store the section_nr in there. IIRC, it's less of an issue with section sizes like 128 MB. > i.e 26 bits in section size, fails to boot. 26 bits would mean 64 MB, no? Not sure if that's possible even without THP (MAX_ORDER - 1, pageblock_order ...) on 64k pages. I'd assume 512 MB is the lowest we can go. I'd assume this would crash :) > > As you have suggested, probably constant defaults (128MB for 4K/16K, 512MB > for 64K) might be better than depending on the CONFIG_FORCE_MAX_ZONEORDER, > at least for now. That's also what I would prefer, keeping it simple. -- Thanks, David / dhildenb