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Date: Thu, 10 Sep 2020 10:02:33 -0300
From: Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table
 folding
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To: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>, 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>, 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 Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:

> As Gerald mentioned, it is very difficult to explain in a clear way.
> Hopefully, one could make sense ot of it.

I would say the page table API requires this invariant:

        pud = pud_offset(p4d, addr);
        do {
		WARN_ON(pud != pud_offset(p4d, addr);
                next = pud_addr_end(addr, end);
        } while (pud++, addr = next, addr != end);

ie pud++ is supposed to be a shortcut for 
  pud_offset(p4d, next)

While S390 does not follow this. Fixing addr_end brings it into
alignment by preventing pud++ from happening.

The only currently known side effect is that gup_fast crashes, but it
sure is an unexpected thing.

This suggests another fix, which is to say that pud++ is undefined and
pud_offset() must always be called, but I think that would cause worse
codegen on all other archs.

Jason