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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D67BAC77B73 for ; Wed, 19 Apr 2023 06:47:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231644AbjDSGro (ORCPT ); Wed, 19 Apr 2023 02:47:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57722 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231159AbjDSGrm (ORCPT ); Wed, 19 Apr 2023 02:47:42 -0400 Received: from out30-101.freemail.mail.aliyun.com (out30-101.freemail.mail.aliyun.com [115.124.30.101]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 31B9D10CE for ; Tue, 18 Apr 2023 23:47:41 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R191e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046060;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0VgTpm5r_1681886857; Received: from 30.97.48.60(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0VgTpm5r_1681886857) by smtp.aliyun-inc.com; Wed, 19 Apr 2023 14:47:37 +0800 Message-ID: <2ef52fb4-ba74-a23e-4f5b-d4725c9fe289@linux.alibaba.com> Date: Wed, 19 Apr 2023 14:47:39 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page() To: Vlastimil Babka , David Hildenbrand , akpm@linux-foundation.org Cc: mgorman@techsingularity.net, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <62e231a8f2e50c04dcadc7a0cfaa6dea5ce1ec05.1681296022.git.baolin.wang@linux.alibaba.com> <94bfa3cc-674e-25b0-e7e2-d74c970acef7@redhat.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/14/2023 11:07 PM, Vlastimil Babka wrote: > On 4/12/23 14:16, Baolin Wang wrote: >> >> >> On 4/12/2023 7:25 PM, David Hildenbrand wrote: >>> On 12.04.23 12:45, Baolin Wang wrote: >>>> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), >>>> which checks whether the given zone contains holes, and uses pfn_valid() >>>> to check if the end pfn is valid. However pfn_valid() can not make sure >>>> the end pfn is not a hole if the size of a pageblock is larger than the >>>> size of a sub-mem_section, since the struct page getting by pfn_to_page() >>>> may represent a hole or an unusable page frame, which may cause incorrect >>>> zone contiguous is set. >>>> >>>> Though another user of pageblock_pfn_to_page() in compaction seems work >>>> well now, it is better to avoid scanning or touching these offline pfns. >>>> So like commit 2d070eab2e82 ("mm: consider zone which is not fully >>>> populated to have holes"), we should also use pfn_to_online_page() for >>>> the end pfn to make sure it is a valid pfn with usable page frame. >>>> Meanwhile the pfn_valid() for end pfn can be dropped now. >>>> >>>> Moreover we've already used pfn_to_online_page() for start pfn to make >>>> sure it is online and valid, so the pfn_valid() for the start pfn is >>>> unnecessary, drop it. >>> >>> pageblocks are supposed to fall into a single memory section, so in mos > cases, if the start is online, so is the end. >> >> Yes, the granularity of memory hotplug is a mem_section. >> >> However, suppose the pageblock order is MAX_ORDER-1, and the size of a >> sub-section is 2M, that means a pageblock will fall into 2 sub >> mem-section, and if there is a hole in the zone, that means the 2nd sub >> mem-section can be invalid without setting subsection_map bitmap. > > Can that really happen? I think the buddy merging in __free_one_page() would > trip on that? I do not think so IIUC. The hole pfns will not free to buddy system, and the buddy system did not change the ms->usage->subsection_map of the hole pfns, which indicates the hole pfns are invalid.