Re: [PATCH 1/4] hugetlb: skip to end of PT page mapping when pte not present

[Baolin Wang]

On 6/18/2022 1:17 AM, Mike Kravetz wrote:
> 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.

Ah, I also overlooked this. Thanks Peter, and thanks Mike for updating.