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[23.128.96.18]) by mx.google.com with ESMTP id c3si5262684edv.329.2021.04.16.09.25.59; Fri, 16 Apr 2021 09:26:22 -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 S239592AbhDPQB0 (ORCPT + 99 others); Fri, 16 Apr 2021 12:01:26 -0400 Received: from foss.arm.com ([217.140.110.172]:45116 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238563AbhDPQAy (ORCPT ); Fri, 16 Apr 2021 12:00:54 -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 DCBA011B3; Fri, 16 Apr 2021 09:00:29 -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 098F13F99C; Fri, 16 Apr 2021 09:00:27 -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, linux-s390@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.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> <10adad00-14de-61b6-ce2a-bdde23a34bcf@csgroup.eu> From: Steven Price Message-ID: Date: Fri, 16 Apr 2021 17:00:22 +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: <10adad00-14de-61b6-ce2a-bdde23a34bcf@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 16:15, Christophe Leroy wrote: > > > Le 16/04/2021 à 17:04, Christophe Leroy a écrit : >> >> >> Le 16/04/2021 à 16:40, Christophe Leroy a écrit : >>> >>> >>> Le 16/04/2021 à 15:00, Steven Price a écrit : >>>> 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 : >>>>>>>> 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. >>>> >>> >>> I'm trying. I reverted the two commits b00ff6d8c and cabe8138. >>> >>> At the moment, I don't get exactly what I expect: For linear memory I >>> get one line for each 8M page whereas before reverting the patches I >>> got one 16M line and one 112M line. >>> >>> And for KASAN shadow area I get two lines for the 2x 8M pages >>> shadowing linear mem then I get one 4M line for each PGDIR entry >>> pointing to kasan_early_shadow_pte. >>> >>> 0xf8000000-0xf87fffff 0x07000000         8M   huge        rw >>> present >>> 0xf8800000-0xf8ffffff 0x07800000         8M   huge        rw >>> present >>> 0xf9000000-0xf93fffff 0x01430000         4M               r >>> present >> ... >>> 0xfec00000-0xfeffffff 0x01430000         4M               r >>> present >>> >>> Any idea ? >>> >> >> >> I think the different with other architectures is here: >> >>      } else if (flag != st->current_flags || level != st->level || >>             addr >= st->marker[1].start_address || >>             pa != st->last_pa + PAGE_SIZE) { >> >> >> In addition to the checks everyone do, powerpc also checks "pa != >> st->last_pa + PAGE_SIZE". >> And it is definitely for that test that page_size argument add been >> added. > > By replacing that test by (pa - st->start_pa != addr - > st->start_address) it works again. So we definitely don't need the real > page size. Yes that should work. Thanks for figuring it out! > >> >> I see that other architectures except RISCV don't dump the physical >> address. But even RISCV doesn't include that check. Yes not having the physical address certainly simplifies things - although I can see why that can be handy to see. The disadvantage is that user space or vmalloc()'d memory will produce a lot of output because the physical addresses are unlikely to be contiguous. And for most uses you don't need the information. >> That physical address dump was added by commit aaa229529244 >> ("powerpc/mm: Add physical address to Linux page table dump") >> [https://github.com/torvalds/linux/commit/aaa2295] >> >> How do other architectures deal with the problem described by the >> commit log of that patch ? AFAIK other architectures are "broken" in this regard. In practice I don't think it often causes an issue though. Steve