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

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Hello Krill,

On Thu, Jun 15, 2017 at 05:52:22PM +0300, Kirill A. Shutemov wrote:
> +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.
> +	 */
> +	{
> +		old = *pmdp;
> +	} while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd);
> +
> +	return old;
> +}

I see further margin for optimization here (although it's only for PAE
x32..).

pmd is stable so we could do:

if (!(pmd & _PAGE_PRESENT)) {
   cast to split_pmd and use xchg on pmd_low like
   native_pmdp_get_and_clear and copy pmd_high non atomically
} else {
  the above cmpxchg64 loop
}

Now thinking about the above I had a second thought if pmdp_establish
is the right interface and if we shouldn't replace pmdp_establish with
pmdp_mknotpresent instead to skip the pmd & _PAGE_PRESENT check that
will always be true in practice, so pmdp_mknotpresent will call
internally pmd_mknotpresent and it won't have to check for pmd &
_PAGE_PRESENT and it would have no cons on x86-64.

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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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