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Mon, 06 Jul 2020 13:56:49 -0400 X-MC-Unique: H6ZBaSZ1Mu2Ux14NzP-5iA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 0A02E80183C; Mon, 6 Jul 2020 17:56:47 +0000 (UTC) Received: from [10.36.112.179] (ovpn-112-179.ams2.redhat.com [10.36.112.179]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6E7B95F7D8; Mon, 6 Jul 2020 17:56:44 +0000 (UTC) Subject: Re: [PATCH v6 07/26] mm: Preserve the PG_arch_* flags in __split_huge_page_tail() To: Catalin Marinas Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Will Deacon , Dave P Martin , Vincenzo Frascino , Szabolcs Nagy , Kevin Brodsky , Andrey Konovalov , Peter Collingbourne , Andrew Morton , Christian Borntraeger References: <20200703153718.16973-1-catalin.marinas@arm.com> <20200703153718.16973-8-catalin.marinas@arm.com> <16aeea8c-b5c4-0d19-2fde-f95ef8dfddc6@redhat.com> <20200706163012.GH28170@gaia> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200706163012.GH28170@gaia> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Rspamd-Queue-Id: 59AE628A4E8 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 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: On 06.07.20 18:30, Catalin Marinas wrote: > On Mon, Jul 06, 2020 at 04:16:13PM +0200, David Hildenbrand wrote: >> On 03.07.20 17:36, Catalin Marinas wrote: >>> When a huge page is split into normal pages, part of the head page fl= ags >>> are transferred to the tail pages. However, the PG_arch_* flags are n= ot >>> part of the preserved set. >>> >>> PG_arch_1 is currently used by the arch code to handle cache maintena= nce >>> for user space (either for I-D cache coherency or for D-cache aliases >>> consistent with the kernel mapping). Since splitting a huge page does >>> not change the physical or virtual address of a mapping, additional >>> cache maintenance for the tail pages is unnecessary. Preserving the >>> PG_arch_1 flag from the head page in the tail pages would not break t= he >>> current use-cases. >> >> ^ is fairly arm64 specific, no? (I remember that the semantics are >> different e.g., on s390x). >=20 > Not entirely arm64 specific. Apart from s390 and x86, I think all the > other architectures use this flag for cache maintenance (I guess they > followed the cachetlb.rst suggestion). My understanding of the s390 and > x86 is that transferring this flag from the head of a compound page to > the tail pages should not cause any issue. We don't even document > anywhere that this flag is meant to disappear on huge page splitting. I > guess no-one noticed because clearing it is relatively benign. On s390x, PG_arch_1 indicates (s390/kernel/uv.c:arch_make_page_accessible= ()) - kernel page tables - for hugetlbfs pages, that storage keys are initialized for that page (IIRC KVM only) - a user space page might be encrypted/secure (KVM only) The latter does not support hugetlbfs/THP. KVM does not support THP. So on s390x the bit should never be set in that context and, therefore, also won't be affected by this change. >=20 > But if there are concerns, I'm happy to guard it with something like > __ARCH_WANT_PG_ARCH_HEAD_TAIL (I need to think of a more suggestive > name). I guess we can avoid that if we properly check+document all users. (ignoring x86 and s390x behavior here might be dangerous, although my gut feeling is that it's ok for both) --=20 Thanks, David / dhildenb