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Fri, 16 Apr 2021 16:40:52 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at c-s.fr Received: from pegase1.c-s.fr ([192.168.12.234]) by localhost (pegase1.c-s.fr [192.168.12.234]) (amavisd-new, port 10024) with ESMTP id 4QkMRAzEi_DN; Fri, 16 Apr 2021 16:40:52 +0200 (CEST) Received: from messagerie.si.c-s.fr (messagerie.si.c-s.fr [192.168.25.192]) by pegase1.c-s.fr (Postfix) with ESMTP id 4FMJmN11tvz9vBnC; Fri, 16 Apr 2021 16:40:52 +0200 (CEST) Received: from localhost (localhost [127.0.0.1]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 781F48B84E; Fri, 16 Apr 2021 16:40:53 +0200 (CEST) X-Virus-Scanned: amavisd-new at c-s.fr Received: from messagerie.si.c-s.fr ([127.0.0.1]) by localhost (messagerie.si.c-s.fr [127.0.0.1]) (amavisd-new, port 10023) with ESMTP id D0sjjGIwO1la; Fri, 16 Apr 2021 16:40:53 +0200 (CEST) Received: from [192.168.4.90] (unknown [192.168.4.90]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 91A338B84C; Fri, 16 Apr 2021 16:40:52 +0200 (CEST) Subject: Re: [PATCH v1 3/5] mm: ptdump: Provide page size to notepage() To: Steven Price , 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: Christophe Leroy Message-ID: Date: Fri, 16 Apr 2021 16:40:52 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr X-Rspamd-Queue-Id: AB155C0007DD X-Stat-Signature: c7cdm6yjm3eu3tejd3zcdbfuhge4jxkw X-Rspamd-Server: rspam02 Received-SPF: none (csgroup.eu>: No applicable sender policy available) receiver=imf22; identity=mailfrom; envelope-from=""; helo=pegase1.c-s.fr; client-ip=93.17.236.30 X-HE-DKIM-Result: none/none X-HE-Tag: 1618584051-5910 Content-Transfer-Encoding: quoted-printable 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: Le 16/04/2021 =C3=A0 15:00, Steven Price a =C3=A9crit=C2=A0: > On 16/04/2021 12:08, Christophe Leroy wrote: >> >> >> Le 16/04/2021 =C3=A0 12:51, Steven Price a =C3=A9crit=C2=A0: >>> On 16/04/2021 11:38, Christophe Leroy wrote: >>>> >>>> >>>> Le 16/04/2021 =C3=A0 11:28, Steven Price a =C3=A9crit=C2=A0: >>>>> 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 >>>>>> --- >>>>>> =C2=A0 arch/arm64/mm/ptdump.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 |=C2=A0 2 +- >>>>>> =C2=A0 arch/riscv/mm/ptdump.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 |=C2=A0 2 +- >>>>>> =C2=A0 arch/s390/mm/dump_pagetables.c |=C2=A0 3 ++- >>>>>> =C2=A0 arch/x86/mm/dump_pagetables.c=C2=A0 |=C2=A0 2 +- >>>>>> =C2=A0 include/linux/ptdump.h=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 |=C2=A0 2 +- >>>>>> =C2=A0 mm/ptdump.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | 16 += +++++++-------- >>>>>> =C2=A0 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 m= m_walk *walk, >>>>>> =C2=A0 { >>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct ptdump_state *st =3D walk->p= rivate; >>>>>> -=C2=A0=C2=A0=C2=A0 st->note_page(st, addr, 4, pte_val(kasan_early= _shadow_pte[0])); >>>>>> +=C2=A0=C2=A0=C2=A0 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=20 >>>>> presents an interesting case here. We short-cut by detecting it's a= KASAN region at a high=20 >>>>> level (PGD/P4D/PUD/PMD) and instead of walking the tree down just c= all note_page() *once* but=20 >>>>> with level=3D=3D4 because we know KASAN sets up the page table like= that. >>>>> >>>>> However the one call actually covers a much larger region - so whil= e PAGE_SIZE matches the=20 >>>>> level it doesn't match the region covered. AFAICT this will lead to= odd results if you enable=20 >>>>> KASAN on powerpc. >>>> >>>> Hum .... I successfully tested it with KASAN, I now realise that I t= ested it with=20 >>>> CONFIG_KASAN_VMALLOC selected. In this situation, since=20 >>>> https://github.com/torvalds/linux/commit/af3d0a686 we don't have any= common shadow page table=20 >>>> 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=20 >>>>> using it purely to find "holes" in the calls to note_page(), but I = haven't worked out why such=20 >>>>> holes would occur. >>>> >>>> I was indeed introduced for KASAN. We have a first commit=20 >>>> https://github.com/torvalds/linux/commit/cabe8138 which uses page si= ze to detect whether it is a=20 >>>> KASAN like stuff. >>>> >>>> Then came https://github.com/torvalds/linux/commit/b00ff6d8c as a fi= x. I can't remember what the=20 >>>> problem was exactly, something around the use of hugepages for kerne= l memory, came as part of=20 >>>> the series=20 >>>> https://patchwork.ozlabs.org/project/linuxppc-dev/cover/cover.158986= 6984.git.christophe.leroy@csgroup.eu/=20 >>> >>> >>> >>> >>> Ah, that's useful context. So it looks like powerpc took a different = route to reducing the KASAN=20 >>> output to x86. >>> >>> Given the generic ptdump code has handling for KASAN already it shoul= d be possible to drop that=20 >>> from the powerpc arch code, which I think means we don't actually nee= d to provide page size to=20 >>> notepage(). Hopefully that means more code to delete ;) >>> >> >> Yes ... and no. >> >> It looks like the generic ptdump handles the case when several pgdir e= ntries points to the same=20 >> kasan_early_shadow_pte. But it doesn't take into account the powerpc c= ase where we have regular=20 >> page tables where several (if not all) PTEs are pointing to the kasan_= early_shadow_page . >=20 > I'm not sure I follow quite how powerpc is different here. But could yo= u have a similar check for=20 > PTEs against kasan_early_shadow_pte as the other levels already have? >=20 > I'm just worried that page_size isn't well defined in this interface an= d it's going to cause=20 > problems in the future. >=20 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=20 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 l= inear mem then I get one 4M=20 line for each PGDIR entry pointing to kasan_early_shadow_pte. 0xf8000000-0xf87fffff 0x07000000 8M huge rw presen= t 0xf8800000-0xf8ffffff 0x07800000 8M huge rw presen= t 0xf9000000-0xf93fffff 0x01430000 4M r presen= t 0xf9400000-0xf97fffff 0x01430000 4M r presen= t 0xf9800000-0xf9bfffff 0x01430000 4M r presen= t 0xf9c00000-0xf9ffffff 0x01430000 4M r presen= t 0xfa000000-0xfa3fffff 0x01430000 4M r presen= t 0xfa400000-0xfa7fffff 0x01430000 4M r presen= t 0xfa800000-0xfabfffff 0x01430000 4M r presen= t 0xfac00000-0xfaffffff 0x01430000 4M r presen= t 0xfb000000-0xfb3fffff 0x01430000 4M r presen= t 0xfb400000-0xfb7fffff 0x01430000 4M r presen= t 0xfb800000-0xfbbfffff 0x01430000 4M r presen= t 0xfbc00000-0xfbffffff 0x01430000 4M r presen= t 0xfc000000-0xfc3fffff 0x01430000 4M r presen= t 0xfc400000-0xfc7fffff 0x01430000 4M r presen= t 0xfc800000-0xfcbfffff 0x01430000 4M r presen= t 0xfcc00000-0xfcffffff 0x01430000 4M r presen= t 0xfd000000-0xfd3fffff 0x01430000 4M r presen= t 0xfd400000-0xfd7fffff 0x01430000 4M r presen= t 0xfd800000-0xfdbfffff 0x01430000 4M r presen= t 0xfdc00000-0xfdffffff 0x01430000 4M r presen= t 0xfe000000-0xfe3fffff 0x01430000 4M r presen= t 0xfe400000-0xfe7fffff 0x01430000 4M r presen= t 0xfe800000-0xfebfffff 0x01430000 4M r presen= t 0xfec00000-0xfeffffff 0x01430000 4M r presen= t Any idea ? Christophe