From: David Hildenbrand <david@redhat.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arch@vger.kernel.org,
Christian Borntraeger <borntraeger@de.ibm.com>,
Szabolcs Nagy <szabolcs.nagy@arm.com>,
Andrey Konovalov <andreyknvl@google.com>,
Kevin Brodsky <kevin.brodsky@arm.com>,
Peter Collingbourne <pcc@google.com>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Will Deacon <will@kernel.org>,
Dave P Martin <Dave.Martin@arm.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v6 07/26] mm: Preserve the PG_arch_* flags in __split_huge_page_tail()
Date: Mon, 6 Jul 2020 19:56:43 +0200 [thread overview]
Message-ID: <bed8d086-8ed5-d72a-7a1f-327ef982d1a5@redhat.com> (raw)
In-Reply-To: <20200706163012.GH28170@gaia>
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 flags
>>> are transferred to the tail pages. However, the PG_arch_* flags are not
>>> part of the preserved set.
>>>
>>> PG_arch_1 is currently used by the arch code to handle cache maintenance
>>> 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 the
>>> current use-cases.
>>
>> ^ is fairly arm64 specific, no? (I remember that the semantics are
>> different e.g., on s390x).
>
> 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.
>
> 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)
--
Thanks,
David / dhildenb
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next prev parent reply other threads:[~2020-07-06 17:58 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-03 15:36 [PATCH v6 00/26] arm64: Memory Tagging Extension user-space support Catalin Marinas
2020-07-03 15:36 ` [PATCH v6 01/26] arm64: mte: system register definitions Catalin Marinas
2020-07-03 15:36 ` [PATCH v6 02/26] arm64: mte: CPU feature detection and initial sysreg configuration Catalin Marinas
2020-07-13 10:08 ` Steven Price
2020-07-13 17:45 ` Catalin Marinas
2020-07-03 15:36 ` [PATCH v6 03/26] arm64: mte: Use Normal Tagged attributes for the linear map Catalin Marinas
2020-07-03 15:36 ` [PATCH v6 04/26] arm64: mte: Add specific SIGSEGV codes Catalin Marinas
2020-07-03 15:36 ` [PATCH v6 05/26] arm64: mte: Handle synchronous and asynchronous tag check faults Catalin Marinas
2020-07-03 15:36 ` [PATCH v6 06/26] mm: Add PG_arch_2 page flag Catalin Marinas
2020-07-06 8:24 ` David Hildenbrand
2020-07-06 11:21 ` Catalin Marinas
2020-07-06 12:42 ` David Hildenbrand
2020-07-03 15:36 ` [PATCH v6 07/26] mm: Preserve the PG_arch_* flags in __split_huge_page_tail() Catalin Marinas
2020-07-06 14:16 ` David Hildenbrand
2020-07-06 16:30 ` Catalin Marinas
2020-07-06 17:56 ` David Hildenbrand [this message]
2020-07-08 12:17 ` Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 08/26] arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 09/26] arm64: mte: Tags-aware copy_{user_, }highpage() implementations Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 10/26] arm64: Avoid unnecessary clear_user_page() indirection Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 11/26] arm64: mte: Tags-aware aware memcmp_pages() implementation Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 12/26] mm: Introduce arch_calc_vm_flag_bits() Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 13/26] arm64: mte: Add PROT_MTE support to mmap() and mprotect() Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 14/26] mm: Introduce arch_validate_flags() Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 15/26] arm64: mte: Validate the PROT_MTE request via arch_validate_flags() Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 16/26] mm: Allow arm64 mmap(PROT_MTE) on RAM-based files Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 17/26] arm64: mte: Allow user control of the tag check mode via prctl() Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 18/26] arm64: mte: Allow user control of the generated random tags " Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 19/26] arm64: mte: Restore the GCR_EL1 register after a suspend Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 20/26] arm64: mte: Add PTRACE_{PEEK,POKE}MTETAGS support Catalin Marinas
2020-07-09 14:41 ` Luis Machado
2020-07-03 15:37 ` [PATCH v6 21/26] fs: Handle intra-page faults in copy_mount_options() Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 22/26] mm: Add arch hooks for saving/restoring tags Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 23/26] arm64: mte: Enable swap of tagged pages Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 24/26] arm64: mte: Save tags when hibernating Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 25/26] arm64: mte: Kconfig entry Catalin Marinas
2020-07-03 15:37 ` [PATCH v6 26/26] arm64: mte: Add Memory Tagging Extension documentation Catalin Marinas
2020-07-09 9:32 ` Szabolcs Nagy
2020-07-09 14:43 ` Catalin Marinas
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