linux-kernel - Re: [RFC PATCH 10/26] hugetlb: add for_each_hgm

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Message-ID: <CAHS8izNO+4Nzg=rgr1R027i2Evo-UKiFEK1gQcMHtF8uxcHzeQ@mail.gmail.com>
Date:   Tue, 28 Jun 2022 14:58:32 -0700
From:   Mina Almasry <almasrymina@...gle.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Jue Wang <juew@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 10/26] hugetlb: add for_each_hgm_shift

On Fri, Jun 24, 2022 at 10:37 AM James Houghton <jthoughton@...gle.com> wrote:
>
> This is a helper macro to loop through all the usable page sizes for a
> high-granularity-enabled HugeTLB VMA. Given the VMA's hstate, it will
> loop, in descending order, through the page sizes that HugeTLB supports
> for this architecture; it always includes PAGE_SIZE.
>
> Signed-off-by: James Houghton <jthoughton@...gle.com>
> ---
>  mm/hugetlb.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 8b10b941458d..557b0afdb503 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6989,6 +6989,16 @@ bool hugetlb_hgm_enabled(struct vm_area_struct *vma)
>         /* All shared VMAs have HGM enabled. */
>         return vma->vm_flags & VM_SHARED;
>  }
> +static unsigned int __shift_for_hstate(struct hstate *h)
> +{
> +       if (h >= &hstates[hugetlb_max_hstate])
> +               return PAGE_SHIFT;

h > &hstates[hugetlb_max_hstate] means that h is out of bounds, no? am
I missing something here?

So is this intending to do:

if (h == hstates[hugetlb_max_hstate]
    return PAGE_SHIFT;

? If so, could we write it as so?

I'm also wondering why __shift_for_hstate(hstate[hugetlb_max_hstate])
== PAGE_SHIFT? Isn't the last hstate the smallest hstate which should
be 2MB on x86? Shouldn't this return PMD_SHIFT in that case?

> +       return huge_page_shift(h);
> +}
> +#define for_each_hgm_shift(hstate, tmp_h, shift) \
> +       for ((tmp_h) = hstate; (shift) = __shift_for_hstate(tmp_h), \
> +                              (tmp_h) <= &hstates[hugetlb_max_hstate]; \
> +                              (tmp_h)++)
>  #endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
>
>  /*
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>