Re: [PATCH v2 16/46] hugetlb: make default arch_make_huge_pte understand small mappings

[James Houghton]

On Wed, Feb 22, 2023 at 1:18 PM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
>
> On Fri, Feb 17, 2023 at 4:28 PM James Houghton <jthoughton@xxxxxxxxxx> wrote:
> >
> > This is a simple change: don't create a "huge" PTE if we are making a
> > regular, PAGE_SIZE PTE. All architectures that want to implement HGM
> > likely need to be changed in a similar way if they implement their own
> > version of arch_make_huge_pte.
> >
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 726d581158b1..b767b6889dea 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -899,7 +899,7 @@ static inline void arch_clear_hugepage_flags(struct page *page) { }
> >  static inline pte_t arch_make_huge_pte(pte_t entry, unsigned int shift,
> >                                        vm_flags_t flags)
> >  {
> > -       return pte_mkhuge(entry);
> > +       return shift > PAGE_SHIFT ? pte_mkhuge(entry) : entry;
> >  }
> >  #endif
> >
>
> How are contig_pte's handled here? Will shift show that it's actually
> a contig_pte and not just PAGE_SHIFT? Or is that arm64 specific so it
> exists only in the arm64 version of this function? Do we need to worry
> about it here?

arm64 implements its own version of arch_make_huge_pte, and 'shift'
does indeed indicate (to arm64) if the PTE is contiguous or not (like
it will be CONT_PTE_SHIFT, for example).