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Date: Tue, 15 Sep 2020 19:09:27 +0200
From: Vasily Gorbik <gor@linux.ibm.com>
Subject: Re: [PATCH v2] mm/gup: fix gup_fast with dynamic page table folding
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To: Andrew Morton <akpm@linux-foundation.org>, Jason Gunthorpe <jgg@ziepe.ca>, John Hubbard <jhubbard@nvidia.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>, Alexander Gordeev <agordeev@linux.ibm.com>, Linus Torvalds <torvalds@linux-foundation.org>, Peter Zijlstra <peterz@infradead.org>, Dave Hansen <dave.hansen@intel.com>, LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, linux-arch <linux-arch@vger.kernel.org>, Russell King <linux@armlinux.org.uk>, Mike Rapoport <rppt@kernel.org>, Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, Michael Ellerman <mpe@ellerman.id.au>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Paul Mackerras <paulus@samba.org>, Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>, Dave Hansen <dave.hansen@linux.intel.com>, Andy Lutomirski <luto@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Arnd Bergmann <arnd@arndb.de>, Andrey Ryabinin <aryabinin@virtuozzo.com>, linux-x86 <x86@kernel.org>, linux-arm <linux-arm-kernel@lists.infradead.org>, linux-power <linuxppc-dev@lists.ozlabs.org>, linux-sparc <sparclinux@vger.kernel.org>, linux-um <linux-um@lists.infradead.org>, linux-s390 <linux-s390@vger.kernel.org>, Heiko Carstens <hca@linux.ibm.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Claudio Imbrenda <imbrenda@linux.ibm.com>

On Fri, Sep 11, 2020 at 10:36:43PM +0200, Vasily Gorbik wrote:
> Currently to make sure that every page table entry is read just once
> gup_fast walks perform READ_ONCE and pass pXd value down to the next
> gup_pXd_range function by value e.g.:
...snip...
> ---
> v2: added brackets &pgd -> &(pgd)
> 
>  arch/s390/include/asm/pgtable.h | 42 +++++++++++++++++++++++----------
>  include/linux/pgtable.h         | 10 ++++++++
>  mm/gup.c                        | 18 +++++++-------
>  3 files changed, 49 insertions(+), 21 deletions(-)

Andrew, any chance you would pick this up?

There is an Ack from Linus. And I haven't seen any objections from Jason or John.
This seems to be as safe for other architectures as possible.

@Jason and John
Any acks/nacks?

Thank you,
Vasily