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 813B6C7618E for ; Mon, 24 Apr 2023 11:20:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231636AbjDXLU6 (ORCPT ); Mon, 24 Apr 2023 07:20:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57318 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230255AbjDXLU4 (ORCPT ); Mon, 24 Apr 2023 07:20:56 -0400 Received: from out30-119.freemail.mail.aliyun.com (out30-119.freemail.mail.aliyun.com [115.124.30.119]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7BAFB30F4 for ; Mon, 24 Apr 2023 04:20:48 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R861e4;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=9;SR=0;TI=SMTPD_---0VgtOkQo_1682335243; Received: from 30.97.48.59(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0VgtOkQo_1682335243) by smtp.aliyun-inc.com; Mon, 24 Apr 2023 19:20:44 +0800 Message-ID: <9a20c0b5-9d8a-2b1d-570a-61c17a4ce5e8@linux.alibaba.com> Date: Mon, 24 Apr 2023 19:20:43 +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 v2 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() To: Michal Hocko Cc: akpm@linux-foundation.org, rppt@kernel.org, ying.huang@intel.com, mgorman@techsingularity.net, vbabka@suse.cz, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <9fc85cce8908938f4fd75ff50bc981c073779aa5.1682229876.git.baolin.wang@linux.alibaba.com> <0733a4cf57109a4136de5ae46fac83fb15bdd528.1682229876.git.baolin.wang@linux.alibaba.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 4/24/2023 5:54 PM, Michal Hocko wrote: > On Sun 23-04-23 18:59:11, 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_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. >> >> 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/ > > Do I remember correctly you've had a specific configuration that would > trigger this case? Yes, I provided an example in previous thread [2] so show the __pageblock_pfn_to_page() is fragile in some cases. [2] https://lore.kernel.org/all/52dfdd2e-9c99-eac4-233e-59919a24323e@linux.alibaba.com/ > >> Signed-off-by: Baolin Wang >> --- >> Changes from v1: >> - Update the comments per Ying and Mike, thanks. >> --- >> mm/page_alloc.c | 7 +++++++ >> 1 file changed, 7 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 6457b64fe562..9756d66f471c 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1502,6 +1502,13 @@ 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 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. >> + * This did not break anything until now, but be careful about this possible >> + * issue when checking whether all pfns of a pageblock are valid. > > It is not really clear what you should be doing (other than to be > careful which is not helpful much TBH) when you encounter this > situation. If the reality changes and this would break in the future > what would breakage look like? What should be done about that? That depends on what the future pfn walkers do, which may access some hole memory with zero-init page frame. For example, if checking the __PageMovable() for a zero-init page frame, that will crash the system. But I can not list all the possible cases. So how about below words? * 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, 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 about this possible * issue when checking whether all pfns of a pageblock are valid, that may * lead to accessing empty page frame, and the worst case can crash the system. * So you should use pfn_to_onlie_page() instead of pfn_valid() to valid the * pfns in a pageblock if such case happens.