On 06/17/22 10:15, Peter Xu wrote:
> Hi, Mike,
>
> On Thu, Jun 16, 2022 at 02:05:15PM -0700, Mike Kravetz wrote:
> > @@ -6877,6 +6896,39 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> > return (pte_t *)pmd;
> > }
> >
> > +/*
> > + * Return a mask that can be used to update an address to the last huge
> > + * page in a page table page mapping size. Used to skip non-present
> > + * page table entries when linearly scanning address ranges. Architectures
> > + * with unique huge page to page table relationships can define their own
> > + * version of this routine.
> > + */
> > +unsigned long hugetlb_mask_last_page(struct hstate *h)
> > +{
> > + unsigned long hp_size = huge_page_size(h);
> > +
> > + switch (hp_size) {
> > + case P4D_SIZE:
> > + return PGDIR_SIZE - P4D_SIZE;
> > + case PUD_SIZE:
> > + return P4D_SIZE - PUD_SIZE;
> > + case PMD_SIZE:
> > + return PUD_SIZE - PMD_SIZE;
> > + default:
>
> Should we add a WARN_ON_ONCE() if it should never trigger?
>
Sure. I will add this.
> > + break; /* Should never happen */
> > + }
> > +
> > + return ~(0UL);
> > +}
> > +
> > +#else
> > +
> > +/* See description above. Architectures can provide their own version. */
> > +__weak unsigned long hugetlb_mask_last_page(struct hstate *h)
> > +{
> > + return ~(0UL);
>
> I'm wondering whether it's better to return 0 rather than ~0 by default.
> Could an arch with !CONFIG_ARCH_WANT_GENERAL_HUGETLB wrongly skip some
> valid address ranges with ~0, or perhaps I misread?
Thank you, thank you, thank you Peter!
Yes, the 'default' return for hugetlb_mask_last_page() should be 0. If
there is no 'optimization', we do not want to modify the address so we
want to OR with 0 not ~0. My bad, I must have been thinking AND instead
of OR.
I will change here as well as in Baolin's patch.
--
Mike Kravetz
