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[23.128.96.18]) by mx.google.com with ESMTP id n5si4648501eji.73.2021.04.16.07.08.42; Fri, 16 Apr 2021 07:09:06 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=arm.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235262AbhDPNBA (ORCPT + 99 others); Fri, 16 Apr 2021 09:01:00 -0400 Received: from foss.arm.com ([217.140.110.172]:40958 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233259AbhDPNBA (ORCPT ); Fri, 16 Apr 2021 09:01:00 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 7604111B3; Fri, 16 Apr 2021 06:00:35 -0700 (PDT) Received: from [192.168.1.179] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9E1B33F85F; Fri, 16 Apr 2021 06:00:33 -0700 (PDT) Subject: Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage() To: Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , akpm@linux-foundation.org Cc: linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-riscv@lists.infradead.org, x86@kernel.org, linux-mm@kvack.org References: <1ef6b954fb7b0f4dfc78820f1e612d2166c13227.1618506910.git.christophe.leroy@csgroup.eu> <41819925-3ee5-4771-e98b-0073e8f095cf@arm.com> <1102cda1-b00f-b6ef-6bf3-22068cc11510@arm.com> <6ff4816b-8ff6-19de-73a2-3fcadc003ccd@csgroup.eu> From: Steven Price Message-ID: Date: Fri, 16 Apr 2021 14:00:28 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <6ff4816b-8ff6-19de-73a2-3fcadc003ccd@csgroup.eu> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16/04/2021 12:08, Christophe Leroy wrote: > > > Le 16/04/2021 à 12:51, Steven Price a écrit : >> On 16/04/2021 11:38, Christophe Leroy wrote: >>> >>> >>> Le 16/04/2021 à 11:28, Steven Price a écrit : >>>> On 15/04/2021 18:18, Christophe Leroy wrote: >>>>> In order to support large pages on powerpc, notepage() >>>>> needs to know the page size of the page. >>>>> >>>>> Add a page_size argument to notepage(). >>>>> >>>>> Signed-off-by: Christophe Leroy >>>>> --- >>>>>   arch/arm64/mm/ptdump.c         |  2 +- >>>>>   arch/riscv/mm/ptdump.c         |  2 +- >>>>>   arch/s390/mm/dump_pagetables.c |  3 ++- >>>>>   arch/x86/mm/dump_pagetables.c  |  2 +- >>>>>   include/linux/ptdump.h         |  2 +- >>>>>   mm/ptdump.c                    | 16 ++++++++-------- >>>>>   6 files changed, 14 insertions(+), 13 deletions(-) >>>>> >>>> [...] >>>>> diff --git a/mm/ptdump.c b/mm/ptdump.c >>>>> index da751448d0e4..61cd16afb1c8 100644 >>>>> --- a/mm/ptdump.c >>>>> +++ b/mm/ptdump.c >>>>> @@ -17,7 +17,7 @@ static inline int note_kasan_page_table(struct >>>>> mm_walk *walk, >>>>>   { >>>>>       struct ptdump_state *st = walk->private; >>>>> -    st->note_page(st, addr, 4, pte_val(kasan_early_shadow_pte[0])); >>>>> +    st->note_page(st, addr, 4, pte_val(kasan_early_shadow_pte[0]), >>>>> PAGE_SIZE); >>>> >>>> I'm not completely sure what the page_size is going to be used for, >>>> but note that KASAN presents an interesting case here. We short-cut >>>> by detecting it's a KASAN region at a high level (PGD/P4D/PUD/PMD) >>>> and instead of walking the tree down just call note_page() *once* >>>> but with level==4 because we know KASAN sets up the page table like >>>> that. >>>> >>>> However the one call actually covers a much larger region - so while >>>> PAGE_SIZE matches the level it doesn't match the region covered. >>>> AFAICT this will lead to odd results if you enable KASAN on powerpc. >>> >>> Hum .... I successfully tested it with KASAN, I now realise that I >>> tested it with CONFIG_KASAN_VMALLOC selected. In this situation, >>> since https://github.com/torvalds/linux/commit/af3d0a686 we don't >>> have any common shadow page table anymore. >>> >>> I'll test again without CONFIG_KASAN_VMALLOC. >>> >>>> >>>> To be honest I don't fully understand why powerpc requires the >>>> page_size - it appears to be using it purely to find "holes" in the >>>> calls to note_page(), but I haven't worked out why such holes would >>>> occur. >>> >>> I was indeed introduced for KASAN. We have a first commit >>> https://github.com/torvalds/linux/commit/cabe8138 which uses page >>> size to detect whether it is a KASAN like stuff. >>> >>> Then came https://github.com/torvalds/linux/commit/b00ff6d8c as a >>> fix. I can't remember what the problem was exactly, something around >>> the use of hugepages for kernel memory, came as part of the series >>> https://patchwork.ozlabs.org/project/linuxppc-dev/cover/cover.1589866984.git.christophe.leroy@csgroup.eu/ >> >> >> >> Ah, that's useful context. So it looks like powerpc took a different >> route to reducing the KASAN output to x86. >> >> Given the generic ptdump code has handling for KASAN already it should >> be possible to drop that from the powerpc arch code, which I think >> means we don't actually need to provide page size to notepage(). >> Hopefully that means more code to delete ;) >> > > Yes ... and no. > > It looks like the generic ptdump handles the case when several pgdir > entries points to the same kasan_early_shadow_pte. But it doesn't take > into account the powerpc case where we have regular page tables where > several (if not all) PTEs are pointing to the kasan_early_shadow_page . I'm not sure I follow quite how powerpc is different here. But could you have a similar check for PTEs against kasan_early_shadow_pte as the other levels already have? I'm just worried that page_size isn't well defined in this interface and it's going to cause problems in the future. Steve