From nobody Thu Feb 12 09:01:38 2026 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 366FAC77B61 for ; Tue, 25 Apr 2023 12:45:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234141AbjDYMpZ (ORCPT ); Tue, 25 Apr 2023 08:45:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55424 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234031AbjDYMpO (ORCPT ); Tue, 25 Apr 2023 08:45:14 -0400 Received: from out30-133.freemail.mail.aliyun.com (out30-133.freemail.mail.aliyun.com [115.124.30.133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D2A2ED307 for ; Tue, 25 Apr 2023 05:45:12 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R121e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045170;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=10;SR=0;TI=SMTPD_---0Vh-TY-f_1682426706; Received: from localhost(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0Vh-TY-f_1682426706) by smtp.aliyun-inc.com; Tue, 25 Apr 2023 20:45:07 +0800 From: Baolin Wang To: akpm@linux-foundation.org Cc: rppt@kernel.org, ying.huang@intel.com, mgorman@techsingularity.net, vbabka@suse.cz, mhocko@suse.com, david@redhat.com, baolin.wang@linux.alibaba.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH v4] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() Date: Tue, 25 Apr 2023 20:44:53 +0800 Message-Id: <5c26368865e79c743a453dea48d30670b19d2e4f.1682425534.git.baolin.wang@linux.alibaba.com> X-Mailer: git-send-email 2.27.0 In-Reply-To: <98fa0a22-77d1-cdb3-1ce2-48a00c3ed5a9@linux.alibaba.com> References: <98fa0a22-77d1-cdb3-1ce2-48a00c3ed5a9@linux.alibaba.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" 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_vali= d() to validate the end pfn. However, the __pageblock_pfn_to_page() 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, 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. See below memory layout as an example and suppose the pageblock order is MAX_ORDER. [ 0.000000] Zone ranges: [ 0.000000] DMA [mem 0x0000000040000000-0x00000000ffffffff] [ 0.000000] DMA32 empty [ 0.000000] Normal [mem 0x0000000100000000-0x0000001fa7ffffff] [ 0.000000] Movable zone start for each node [ 0.000000] Early memory node ranges [ 0.000000] node 0: [mem 0x0000000040000000-0x0000001fa3c7ffff] [ 0.000000] node 0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff] [ 0.000000] node 0: [mem 0x0000001fa4000000-0x0000001fa402ffff] [ 0.000000] node 0: [mem 0x0000001fa4030000-0x0000001fa40effff] [ 0.000000] node 0: [mem 0x0000001fa40f0000-0x0000001fa73cffff] [ 0.000000] node 0: [mem 0x0000001fa73d0000-0x0000001fa745ffff] [ 0.000000] node 0: [mem 0x0000001fa7460000-0x0000001fa746ffff] [ 0.000000] node 0: [mem 0x0000001fa7470000-0x0000001fa758ffff] [ 0.000000] node 0: [mem 0x0000001fa7590000-0x0000001fa7dfffff] Focus on the last memory range, and there is a hole for the range [mem 0x0000001fa7590000-0x0000001fa7dfffff]. That means the last pageblock will contain the range from 0x1fa7c00000 to 0x1fa7ffffff, since the pageblock must be 4M aligned. And in this pageblock, these pfns will fall into 2 sub-section (the sub-section size is 2M aligned). So, the 1st sub-section (indicates pfn range: 0x1fa7c00000 - 0x1fa7dfffff ) in this pageblock is valid by calling subsection_map_init() in free_area_init(), but the 2nd sub-section (indicates pfn range: 0x1fa7e00000 - 0x1fa7ffffff ) in this pageblock is not 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 Acked-by: Michal Hocko Reviewed-by: "Huang, Ying" --- Changes from v3: - Update the comments to make it clear. - Add acked tag from Michal. Changes from v2: - Update the commit log and comments per Michal, thanks. Changes from v1: - Update the comments per Ying and Mike, thanks. --- mm/page_alloc.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 6457b64fe562..af9c995d3c1e 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1502,6 +1502,15 @@ void __free_pages_core(struct page *page, unsigned i= nt order) * interleaving within a single pageblock. It is therefore sufficient to c= heck * the first and last page of a pageblock and avoid checking each individu= al * page in a pageblock. + * + * Note: the function may return non-NULL struct page even for a page block + * which contains a memory hole (i.e. there is no physical memory for a su= bset + * of the pfn range). For example, if the pageblock order is MAX_ORDER, wh= ich + * 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 should be safe most= of + * the time because struct pages are still initialized via init_unavailabl= e_range() + * and pfn walkers shouldn't touch any physical memory range for which the= y do + * not recognize any specific metadata in struct pages. */ struct page *__pageblock_pfn_to_page(unsigned long start_pfn, unsigned long end_pfn, struct zone *zone) --=20 2.27.0