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

[Mike Kravetz]

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