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Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper

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On Mon, Jun 19, 2017 at 04:22:29PM +0100, Catalin Marinas wrote:
> Hi Kirill,
> 
> On Thu, Jun 15, 2017 at 05:52:22PM +0300, Kirill A. Shutemov wrote:
> > We need an atomic way to setup pmd page table entry, avoiding races with
> > CPU setting dirty/accessed bits. This is required to implement
> > pmdp_invalidate() that doesn't loose these bits.
> > 
> > On PAE we have to use cmpxchg8b as we cannot assume what is value of new pmd and
> > setting it up half-by-half can expose broken corrupted entry to CPU.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: H. Peter Anvin <hpa@zytor.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> 
> I'll look at this from the arm64 perspective. It would be good if we can
> have a generic atomic implementation based on cmpxchg64 but I need to
> look at the details first.

Unfortunately, I'm not sure it's possbile.

The format of a page table is defined per-arch. We cannot assume much about
it in generic code.

I guess we could make it compile by casting to 'unsigned long', but is it
useful?
Every architecture manintainer still has to validate that this assumption
is valid for the architecture.

> > +static inline pmd_t pmdp_establish(pmd_t *pmdp, pmd_t pmd)
> > +{
> > +	pmd_t old;
> > +
> > +	/*
> > +	 * We cannot assume what is value of pmd here, so there's no easy way
> > +	 * to set if half by half. We have to fall back to cmpxchg64.
> > +	 */
> > +	{
> 
> BTW, you are missing a "do" here (and it probably compiles just fine
> without it, though different behaviour).

Ouch. Thanks.

Hm, what is semantics of the construct without a "do"?

> 
> > +		old = *pmdp;
> > +	} while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd);
> > +
> > +	return old;
> > +}
> 
> -- 
> Catalin
> 
> --
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-- 
 Kirill A. Shutemov

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[PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2017-06-15 17:00 +0200
  Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper Andrea Arcangeli <aarcange@redhat.com> - 2017-06-16 15:40 +0200
    Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-06-19 14:50 +0200
  Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper Martin Schwidefsky <schwidefsky@de.ibm.com> - 2017-06-19 07:50 +0200
    Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-06-19 14:50 +0200
      Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper Martin Schwidefsky <schwidefsky@de.ibm.com> - 2017-06-19 15:10 +0200
  Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper Catalin Marinas <catalin.marinas@arm.com> - 2017-06-19 17:30 +0200
    Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-06-19 18:10 +0200
      Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper Catalin Marinas <catalin.marinas@arm.com> - 2017-06-19 19:10 +0200
        Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-06-20 00:00 +0200
  Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper Nadav Amit <nadav.amit@gmail.com> - 2017-06-19 19:20 +0200
    Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-06-20 00:00 +0200

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