Re: [PATCH] mm/hugetlb: allow gigantic page allocation to migrate away smaller huge page

[Pingfan Liu]

On Mon, Jun 24, 2019 at 1:03 PM Ira Weiny <ira.weiny@xxxxxxxxx> wrote:
>
> On Mon, Jun 24, 2019 at 12:21:08PM +0800, Pingfan Liu wrote:
> > The current pfn_range_valid_gigantic() rejects the pud huge page allocation
> > if there is a pmd huge page inside the candidate range.
> >
> > But pud huge resource is more rare, which should align on 1GB on x86. It is
> > worth to allow migrating away pmd huge page to make room for a pud huge
> > page.
> >
> > The same logic is applied to pgd and pud huge pages.
>
> I'm sorry but I don't quite understand why we should do this.  Is this a bug or
> an optimization?  It sounds like an optimization.
Yes, an optimization. It can help us to success to allocate a 1GB
hugetlb if there is some 2MB hugetlb sit in the candidate range.
Allocation 1GB hugetlb requires more tough condition, not only a
continuous 1GB range, but also aligned on GB. While allocating a 2MB
range is easier.
>
> >
> > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >  mm/hugetlb.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index ac843d3..02d1978 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1081,7 +1081,11 @@ static bool pfn_range_valid_gigantic(struct zone *z,
> >                       unsigned long start_pfn, unsigned long nr_pages)
> >  {
> >       unsigned long i, end_pfn = start_pfn + nr_pages;
> > -     struct page *page;
> > +     struct page *page = pfn_to_page(start_pfn);
> > +
> > +     if (PageHuge(page))
> > +             if (compound_order(compound_head(page)) >= nr_pages)
>
> I don't think you want compound_order() here.
Yes, your are right.

Thanks,
  Pingfan
>
> Ira
>
> > +                     return false;
> >
> >       for (i = start_pfn; i < end_pfn; i++) {
> >               if (!pfn_valid(i))
> > @@ -1098,8 +1102,6 @@ static bool pfn_range_valid_gigantic(struct zone *z,
> >               if (page_count(page) > 0)
> >                       return false;
> >
> > -             if (PageHuge(page))
> > -                     return false;
> >       }
> >
> >       return true;
> > --
> > 2.7.5
> >