linux-kernel - Re: [PATCH v2 16/46] hugetlb: make default arch_make_huge

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Message-ID: <CADrL8HUhD2aHsXw06chQCcCxnJyw7HnD_rE7kK=5X3qzQrVL8Q@mail.gmail.com>
Date:   Wed, 22 Feb 2023 14:52:18 -0800
From:   James Houghton <jthoughton@...gle.com>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Zach O'Keefe" <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Frank van der Linden <fvdl@...gle.com>,
        Jiaqi Yan <jiaqiyan@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 16/46] hugetlb: make default arch_make_huge_pte
 understand small mappings

On Wed, Feb 22, 2023 at 1:18 PM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Fri, Feb 17, 2023 at 4:28 PM James Houghton <jthoughton@...gle.com> 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@...gle.com>
> >
> > 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).