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SubjectRe: [PATCH] mm/hugetlb: allow gigantic page allocation to migrate away smaller huge page
On Mon, Jun 24, 2019 at 1:03 PM Ira Weiny <ira.weiny@intel.com> 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@gmail.com>
> > Cc: Mike Kravetz <mike.kravetz@oracle.com>
> > Cc: Oscar Salvador <osalvador@suse.de>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: linux-kernel@vger.kernel.org
> > ---
> > 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
> >

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