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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 17 Apr 2019 09:57:01 -0700
Message-ID: <CAHk-=wj5j-zMo7hVNxiw3+5L2d1WQVm3x4fbpg=OXLzDh8FhYg@mail.gmail.com>
Subject: Re: Linux 5.1-rc5
To:     Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org,
        linux-s390 <linux-s390@vger.kernel.org>
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On Wed, Apr 17, 2019 at 1:02 AM Martin Schwidefsky
<schwidefsky@de.ibm.com> wrote:
>
> Grumpf, that does *not* work. For gup the table entries may be read only
> once. Now I remember why I open-coded p4d_offset, pud_offset and pmd_offset
> in arch/s390/mm/gup.c, to avoid to read the table entries twice.
> It will be hard to use the common gup code after all.

Hmm. The common gup code generally should do the "read only once"
thing too (since by definition the gup-fast case is done without
locking), although it's probably the case that most architectures
simply don't care.

What would it require for the generic code to work for s390?

                    Linus