Re: [RFC PATCH 10/26] hugetlb: add for_each_hgm_shift

[Mike Kravetz <mike.kravetz@oracle.com>]

On 06/28/22 14:58, Mina Almasry wrote:
> On Fri, Jun 24, 2022 at 10:37 AM James Houghton <jthoughton@google.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@google.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?
> 

I too am missing how this is working for similar reasons.
-- 
Mike Kravetz

> > +       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
> >