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Date: Wed, 9 Sep 2020 15:03:24 -0300
From: Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table
 folding
Message-ID: <20200909180324.GI87483@ziepe.ca>
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To: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Dave Hansen <dave.hansen@intel.com>, John Hubbard <jhubbard@nvidia.com>, LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, linux-arch <linux-arch@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.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>, Peter Zijlstra <peterz@infradead.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>, Alexander Gordeev <agordeev@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>, Heiko Carstens <hca@linux.ibm.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Claudio Imbrenda <imbrenda@linux.ibm.com>

On Wed, Sep 09, 2020 at 07:25:34PM +0200, Gerald Schaefer wrote:
> I actually had to draw myself a picture to get some hold of
> this, or rather a walk-through with a certain pud-crossing
> range in a folded 3-level scenario. Not sure if I would have
> understood my explanation above w/o that, but I hope you can
> make some sense out of it. Or draw yourself a picture :-)

What I don't understand is how does anything work with S390 today?

If the fix is only to change pxx_addr_end() then than generic code
like mm/pagewalk.c will iterate over a *different list* of page table
entries. 

It's choice of entries to look at is entirely driven by pxx_addr_end().

Which suggest to me that mm/pagewalk.c also doesn't work properly
today on S390 and this issue is not really about stack variables?

Fundamentally if pXX_offset() and pXX_addr_end() must be consistent
together, if pXX_offset() is folded then pXX_addr_end() must cause a
single iteration of that level.

Jason