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 E46C5C77B78 for ; Sun, 23 Apr 2023 01:27:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230079AbjDWB1j (ORCPT ); Sat, 22 Apr 2023 21:27:39 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59906 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230044AbjDWB1f (ORCPT ); Sat, 22 Apr 2023 21:27:35 -0400 Received: from out30-111.freemail.mail.aliyun.com (out30-111.freemail.mail.aliyun.com [115.124.30.111]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E62FA10EC for ; Sat, 22 Apr 2023 18:27:32 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R631e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046049;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0VghjePw_1682213247; Received: from 30.97.48.67(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0VghjePw_1682213247) by smtp.aliyun-inc.com; Sun, 23 Apr 2023 09:27:28 +0800 Message-ID: <1e4ca204-6813-a809-4361-4197103814b0@linux.alibaba.com> Date: Sun, 23 Apr 2023 09:27:28 +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 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() To: "Huang, Ying" Cc: akpm@linux-foundation.org, mgorman@techsingularity.net, vbabka@suse.cz, mhocko@suse.com, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <02defcbe9d7a797a2257e5f6a28ff7ea78e394e5.1682158312.git.baolin.wang@linux.alibaba.com> <87cz3vs8nn.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Baolin Wang In-Reply-To: <87cz3vs8nn.fsf@yhuang6-desk2.ccr.corp.intel.com> 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 4/23/2023 9:13 AM, Huang, Ying wrote: > Baolin Wang writes: > >> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), which >> checks whether the given zone contains holes, and uses pfn_to_online_page() >> to validate if the start pfn is online and valid, as well as using pfn_valid() >> to validate the end pfn. >> >> However, though the start pfn of a pageblock is valid, it can not always >> guarantee the end pfn of the pageblock is also valid (may be holes) in some >> cases. For example, if the pageblock order is MAX_ORDER - 1, which will fall >> into 2 sub-sections, and the end pfn of the pageblock may be hole even though >> the start pfn is online and valid. >> >> This did not break anything until now, but the zone continuous is fragile >> in this possible scenario. So as previous discussion[1], it is better to >> add some comments to explain this possible issue in case there are some >> future pfn walkers that rely on this. >> >> [1] https://lore.kernel.org/all/87r0sdsmr6.fsf@yhuang6-desk2.ccr.corp.intel.com/ >> >> Signed-off-by: Baolin Wang >> --- >> mm/page_alloc.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 6457b64fe562..dc4005b32ae0 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1502,6 +1502,14 @@ void __free_pages_core(struct page *page, unsigned int order) >> * interleaving within a single pageblock. It is therefore sufficient to check >> * the first and last page of a pageblock and avoid checking each individual >> * page in a pageblock. >> + * >> + * Note: if the start pfn of a pageblock is valid, but it can not always guarantee >> + * the end pfn of the pageblock is also valid (may be holes) in some cases. For > > "valid" sounds confusing here. pfn_valid() is true, but the pfn is > considered invalid at some degree. How about the following? > > Note: the function may return non-NULL even if the end pfn of a > pageblock is in a memory hole in some situations. For > >> + * example, if the pageblock order is MAX_ORDER - 1, which will fall into 2 >> + * sub-sections, and the end pfn of the pageblock may be hole even though the >> + * start pfn is online and valid. This did not break anything until now, but be >> + * careful this possible issue when checking if the whole pfns are valid of a > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > whether all pfns of a pageblock are valid. ? > >> + * pageblock. >> */ >> struct page *__pageblock_pfn_to_page(unsigned long start_pfn, >> unsigned long end_pfn, struct zone *zone) > > My English is poor. So, feel free to ignore the comments. Better than me:) . Will do in next version. Thanks.