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Date:   Tue, 23 Apr 2019 17:38:59 +0200
From:   Martin Schwidefsky <schwidefsky@de.ibm.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
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        linux-s390 <linux-s390@vger.kernel.org>
Subject: Re: Linux 5.1-rc5
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On Fri, 19 Apr 2019 10:27:17 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, Apr 19, 2019 at 6:33 AM Martin Schwidefsky
> <schwidefsky@de.ibm.com> wrote:
> >
> > That problem got stuck in my head and I thought more about it. Why not
> > emulate the static folding sequence in the s390 page table code?  
> 
> So this model seems much closer to what x86 does in its folding, where
> the pattern is basically
> 
> > static inline pX-1d_t *pXd_offset(pXd_t *pXd, unsigned long address)
> > {
> >         if (pXd_folded(pXd)
> >                 return (pX-1d_t *) pXd;
> >         return (pX-1d_t *) pXd_deref(*pXd) + pXd_index(address);
> > }  
> 
> which is really how the code is designed to work (ie the folded entry
> doesn't actually do anything to the page directory pointer, it just
> says "ok, we'll use this exact page directory pointer for the next
> lower level instead".
> 
> And that's very much what allows the generic gup code to load the
> entry once, and use a temporary, and as you walk down the chain, if it
> is folded it just then uses that (previous) temporary value for the
> next level instead. IOW, the lower level page table is hidden inside
> the upper level one, and folding just means "don't do any offsets,
> don't change any values, just use the entry as-is for the next lower
> level".
> 
> So I think that's the right thing to do.

Ok, I added two patches for my s390/linux:features branch

Martin Schwidefsky (2):
      s390/mm: make the pxd_offset functions more robust
      s390/mm: convert to the generic get_user_pages_fast code

All code changes are inside arch/s390, I plan to include these patches with
the next merge window. That gives us a little bit of time to run our tests.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.