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

[Andrea Arcangeli]

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.