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SubjectRe: [RFC PATCH 10/26] hugetlb: add for_each_hgm_shift
On Tue, Jun 28, 2022 at 2:58 PM Mina Almasry <almasrymina@google.com> 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?

Yeah, it goes out of bounds intentionally. Maybe I should have called
this out. We need for_each_hgm_shift to include PAGE_SHIFT, and there
is no hstate for it. So to handle it, we iterate past the end of the
hstate array, and when we are past the end, we return PAGE_SHIFT and
stop iterating further. This is admittedly kind of gross; if you have
other suggestions for a way to get a clean `for_each_hgm_shift` macro
like this, I'm all ears. :)

>
> So is this intending to do:
>
> if (h == hstates[hugetlb_max_hstate]
> return PAGE_SHIFT;
>
> ? If so, could we write it as so?

Yeah, this works. I'll write it this way instead. If that condition is
true, `h` is out of bounds (`hugetlb_max_hstate` is past the end, not
the index for the final element). I guess `hugetlb_max_hstate` is a
bit of a misnomer.

>
> 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?

`huge_page_shift(hstate[hugetlb_max_hstate-1])` is PMD_SHIFT on x86.
Actually reading `hstate[hugetlb_max_hstate]` would be bad, which is
why `__shift_for_hstate` exists: to return PAGE_SIZE when we would
otherwise attempt to compute
`huge_page_shift(hstate[hugetlb_max_hstate])`.

>
> > + 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]; \

Note the <= here. If we wanted to always remain inbounds here, we'd
want < instead. But we don't have an hstate for PAGE_SIZE.

> > + (tmp_h)++)
> > #endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
> >
> > /*
> > --
> > 2.37.0.rc0.161.g10f37bed90-goog
> >

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