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From:   Pingfan Liu <kernelfans@gmail.com>
Date:   Mon, 24 Jun 2019 14:10:02 +0800
Message-ID: <CAFgQCTs14R5P7RpCTMwLCMJrGgPzbTGp4tvxCJA0kFgD8_y==g@mail.gmail.com>
Subject: Re: [PATCH] mm/hugetlb: allow gigantic page allocation to migrate
 away smaller huge page
To:     Anshuman Khandual <anshuman.khandual@arm.com>
Cc:     linux-mm@kvack.org, Mike Kravetz <mike.kravetz@oracle.com>,
        Oscar Salvador <osalvador@suse.de>,
        David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jun 24, 2019 at 1:16 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
>
>
> On 06/24/2019 09:51 AM, 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.
>
> The huge page in the range can either be a THP or HugeTLB and migrating them has
> different costs and chances of success. THP migration will involve splitting if
> THP migration is not enabled and all related TLB related costs. Are you sure
> that a PUD HugeTLB allocation really should go through these ? Is there any
PUD hugetlb has already driven out PMD thp in current. This patch just
want to make PUD hugetlb survives PMD hugetlb.

> guarantee that after migration of multiple PMD sized THP/HugeTLB pages on the
> given range, the allocation request for PUD will succeed ?
The migration is complicated, but as my understanding, if there is no
gup pin in the range and there is enough memory including swap, then
it will success.
>
> >
> > 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)
> > +                     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;
> >
>
> So except in the case where there is a bigger huge page in the range this will
> attempt migrating everything on the way. As mentioned before if it all this is
> a good idea, it needs to differentiate between HugeTLB and THP and also take
> into account costs of migrations and chance of subsequence allocation attempt
> into account.
Sorry, but I think this logic is only for hugetlb. The caller
alloc_gigantic_page() is only used inside mm/hugetlb.c, not by
huge_memory.c.

Thanks,
  Pingfan