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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 127A1C77B61 for ; Tue, 25 Apr 2023 02:33:22 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 424426B0071; Mon, 24 Apr 2023 22:33:22 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 3ADEF6B0074; Mon, 24 Apr 2023 22:33:22 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 24DBA6B0075; Mon, 24 Apr 2023 22:33:22 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 0FA1F6B0071 for ; Mon, 24 Apr 2023 22:33:22 -0400 (EDT) Received: from smtpin01.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 1F3F8ACC5B for ; Tue, 25 Apr 2023 02:33:21 +0000 (UTC) X-FDA: 80718341802.01.C580436 Received: from mga11.intel.com (mga11.intel.com [192.55.52.93]) by imf19.hostedemail.com (Postfix) with ESMTP id 098981A0003 for ; Tue, 25 Apr 2023 02:33:17 +0000 (UTC) Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=P+WMxXGL; spf=pass (imf19.hostedemail.com: domain of ying.huang@intel.com designates 192.55.52.93 as permitted sender) smtp.mailfrom=ying.huang@intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1682389999; a=rsa-sha256; cv=none; b=7149aWG0WiZGuHIfaQJW5gDMB/Fs5rsghEljI8nEgCH4JRKqjIdQt6zW1gb2yRY6o54rBq Q+w+hiBMg+pYQfCmuvEzypixjsEs1juLmotaLOVnE00uHRWcT3qwzuFW1ucsF3FKCy834z Iijb7d/L7ZEO8FCH3KSrnu6l11Ufkoo= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=P+WMxXGL; spf=pass (imf19.hostedemail.com: domain of ying.huang@intel.com designates 192.55.52.93 as permitted sender) smtp.mailfrom=ying.huang@intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1682389999; h=from:from:sender: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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=2WL4JqLvbSE/zGYR8Oqiq1TnB0QbKOPL58n28wbCQ04=; b=OvW2mzUPOmMJCVO5VTXWDZYw29luX42bJHSTqHbJtmFjNscOf81JU2wezNgQRsk1ubskYj KeU/UcIcfi2ZHwXFTIbK6NZJVWKsrr25UXI6uQ7qXsvVAL1QPmzA7hqmc95KNPe8/kygcE JqKTs5Ci4ON0nFsF0azL8zL9lmaMeMA= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1682389998; x=1713925998; h=from:to:cc:subject:references:date:in-reply-to: message-id:mime-version; bh=ZPnkc0Cj5PMJtph+7kON5XI90+RTkXdeREZNVB/7g6M=; b=P+WMxXGLc/H6bsnDZXQPLfWUdWE9EcZtkFy9/CkB1tShHkA4Wc0W5OYZ KHGO/DnzbPsylkbJi5Gujc0sFNPT/ZdyYutl7oHohlfFRsayxy/LIiwag HkjKOJUknv9nBRqx6fv7Q87hpQz5vBhGnBWyfHUw67qXIEKrY5iNT91JC hVsR9Gl5tBB1PPF3H6d2U8K8Ses2zZgmqwbNzwCd0Kc9yt1J3YpaDZY// v7HFQ6bWYcz3Rq5/0TQTChqhiwVZrIDUBPmBUM+WrDfOsQ0PejN4PS8tr TcZ57giJac+FiJjiO3CXRsRH5mPR8d8BAf+387JTbaoVo43mmu6JpnZ6D A==; X-IronPort-AV: E=McAfee;i="6600,9927,10690"; a="344109429" X-IronPort-AV: E=Sophos;i="5.99,224,1677571200"; d="scan'208";a="344109429" Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Apr 2023 19:33:15 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10690"; a="867709552" X-IronPort-AV: E=Sophos;i="5.99,224,1677571200"; d="scan'208";a="867709552" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Apr 2023 19:33:13 -0700 From: "Huang, Ying" To: Baolin Wang Cc: , , , , , , , Subject: Re: [PATCH v3 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() References: <50b5e05dbb007e3a969ac946bc9ee0b2b77b185f.1682342634.git.baolin.wang@linux.alibaba.com> <87zg6wkdy8.fsf@yhuang6-desk2.ccr.corp.intel.com> <060aab79-8170-56ad-797d-9d339f6c0b61@linux.alibaba.com> Date: Tue, 25 Apr 2023 10:30:30 +0800 In-Reply-To: <060aab79-8170-56ad-797d-9d339f6c0b61@linux.alibaba.com> (Baolin Wang's message of "Tue, 25 Apr 2023 09:27:23 +0800") Message-ID: <87r0s8k81l.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspam-User: X-Rspamd-Queue-Id: 098981A0003 X-Rspamd-Server: rspam01 X-Stat-Signature: mf3mabo1jjeq4aiegoia4txpma4kuczq X-HE-Tag: 1682389997-339519 X-HE-Meta: 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 qXk50c8c nE96diFue9NnjUh8pQHY9yb/uEwYkJfa2kHAEKjiBgXDzIrMBM+MFMln6lNIBHLOVo0/hdlSflPXar3rDrKJKVqZXQzYarXeP8xb417oNVao0fcNbCpeTRJ1GpQohmlS5501CRGzqVwt9zJjUCz0oE8kAsRlZx1yI396CITvAdlPH5PA3sgWAKlbIdkKvre+PYdwoUxKe1TlvUc2JnZYlBUNX/NOHfrN8OZEoDPSmGOtdDdNfRaZJ5uMDGuWZOLVioUkrh5MVSU0BNIj/k2GBJgISCsMWuS9/J8v83izlX+7QDoQSBW/M71xa4EFVeNc+craAtQjSDF4U3uAUO84MOJd8keALXAyBpEiMyZZ/+hAtF3c= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Baolin Wang writes: > On 4/25/2023 8:22 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, 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 >>> --- >>> Changes from v2: >>> - Update the commit log and comments per Michal, thanks. >>> Changes from v1: >>> - Update the comments per Ying and Mike, thanks. >>> >>> Note, I did not add Huang Ying's reviewed tag, since there are some >>> updates per Michal's suggestion. Ying, please review the v3. 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..bd124390c79b 100644 >>> --- a/mm/page_alloc.c >>> +++ b/mm/page_alloc.c >>> @@ -1502,6 +1502,15 @@ 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: 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 subset >>> + * of the pfn range). 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. This should be safe most of >>> + * the time because struct pages are still zero pre-filled and pfn walkers >> I don't think the pfn is just zero-filled even it's a hole. Can you >> confirm that? In memmap_init() and memmap_init_zone_range(), >> init_unavailable_range() is called to initialize the struct page. > > Yes, what I mean is the page frames were initialized to zero firstly, > and some fields were initialized to default value. The "zero > pre-filled" seems confusing, may be change to "initialized"? Yes. That sounds good. Best Regards, Huang, Ying