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 6A0C6C43334 for ; Tue, 21 Jun 2022 08:49:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348251AbiFUItr (ORCPT ); Tue, 21 Jun 2022 04:49:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35898 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230049AbiFUItq (ORCPT ); Tue, 21 Jun 2022 04:49:46 -0400 Received: from out30-132.freemail.mail.aliyun.com (out30-132.freemail.mail.aliyun.com [115.124.30.132]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0CF882610D for ; Tue, 21 Jun 2022 01:49:44 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045168;MF=xianting.tian@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0VH0a80n_1655801380; Received: from B-LB6YLVDL-0141.local(mailfrom:xianting.tian@linux.alibaba.com fp:SMTPD_---0VH0a80n_1655801380) by smtp.aliyun-inc.com; Tue, 21 Jun 2022 16:49:41 +0800 Subject: Re: [PATCH] mm: fixup validation of buddy pfn To: David Hildenbrand , akpm@linux-foundation.org, ziy@nvidia.com Cc: guoren@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20220621031118.3650529-1-xianting.tian@linux.alibaba.com> From: Xianting Tian Message-ID: <502ccdeb-372c-1d05-4b5e-a962f39cfb55@linux.alibaba.com> Date: Tue, 21 Jun 2022 16:49:40 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2022/6/21 下午4:01, David Hildenbrand 写道: > On 21.06.22 05:11, Xianting Tian wrote: >> For RISC-V arch the first 2MB RAM could be reserved for opensbi, >> and the arch code may don't create pages for the first 2MB RAM, >> so it would have pfn_base=512 and mem_map began with 512th PFN when >> CONFIG_FLATMEM=y. >> >> But __find_buddy_pfn algorithm thinks the start PFN 0, it could get >> 0 PFN or less than the pfn_base value, so page_is_buddy() can't >> verify the page whose PFN is 0 ~ 511, actually we don't have valid >> pages for PFN 0 ~ 511. >> >> Actually, buddy system should not assume Arch cretaed pages for >> reserved memory, Arch may don't know the implied limitation. > Ehm, sorry, no. Archs have to stick to the rules of the buddy, not the > other way around. Why should we add additional overhead to the buddy > just because arch XYZ wants to be special? We ever sent a patch to create mapping for the first 2MB RAM for RISC-V, But it is not accetped. But I am just wondering, if we have the RAM whose physical base address is not 0, for example, start with 0x200000(2Mb). Then the base PFN is (0x200000 >> 12) = 512, Do we still need to create mapping for the non-existing first 2Mb RAM, if not, the issue still exist under the case? > > If at all, we should fail hard if an arch doesn't play with the rules > and make this a VM_BUG_ON(). > >> With this patch, we can gurantee a valid buddy no matter what we >> have pages for reserved memory or not. >> >> Fixes: 8170ac4700d26f65 ("mm: wrap __find_buddy_pfn() with a necessary buddy page validation") >> Signed-off-by: Xianting Tian >> --- >> mm/internal.h | 6 +++++- >> 1 file changed, 5 insertions(+), 1 deletion(-) >> >> diff --git a/mm/internal.h b/mm/internal.h >> index c0f8fbe0445b..0ec446caeb2e 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -322,7 +322,8 @@ __find_buddy_pfn(unsigned long page_pfn, unsigned int order) >> * The found buddy can be a non PageBuddy, out of @page's zone, or its order is >> * not the same as @page. The validation is necessary before use it. >> * >> - * Return: the found buddy page or NULL if not found. >> + * Return: the found buddy page or NULL if not found or NULL if buddy pfn is >> + * not valid. >> */ >> static inline struct page *find_buddy_page_pfn(struct page *page, >> unsigned long pfn, unsigned int order, unsigned long *buddy_pfn) >> @@ -330,6 +331,9 @@ static inline struct page *find_buddy_page_pfn(struct page *page, >> unsigned long __buddy_pfn = __find_buddy_pfn(pfn, order); >> struct page *buddy; >> >> + if (!pfn_valid(__buddy_pfn)) >> + return NULL; >> + >> buddy = page + (__buddy_pfn - pfn); >> if (buddy_pfn) >> *buddy_pfn = __buddy_pfn; >