linux-kernel - Re: [RFC PATCH 19/26] hugetlb: add HGM support for copy_hugetlb_page

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Message-ID: <CADrL8HVimb9qx8fjhgFMfj2hWwD3+6_ZY7W=gHbf6as6qbiLYQ@mail.gmail.com>
Date:   Tue, 12 Jul 2022 10:19:59 -0700
From:   James Houghton <jthoughton@...gle.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Jue Wang <juew@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 19/26] hugetlb: add HGM support for copy_hugetlb_page_range

On Mon, Jul 11, 2022 at 4:41 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 06/24/22 17:36, James Houghton wrote:
> > This allows fork() to work with high-granularity mappings. The page
> > table structure is copied such that partially mapped regions will remain
> > partially mapped in the same way for the new process.
> >
> > Signed-off-by: James Houghton <jthoughton@...gle.com>
> > ---
> >  mm/hugetlb.c | 74 +++++++++++++++++++++++++++++++++++++++++-----------
> >  1 file changed, 59 insertions(+), 15 deletions(-)
>
> FYI -
> With https://lore.kernel.org/linux-mm/20220621235620.291305-5-mike.kravetz@oracle.com/
> copy_hugetlb_page_range() should never be called for shared mappings.
> Since HGM only works on shared mappings, code in this patch will never
> be executed.
>
> I have a TODO to remove shared mapping support from copy_hugetlb_page_range.

Thanks Mike. If I understand things correctly, it seems like I don't
have to do anything to support fork() then; we just don't copy the
page table structure from the old VMA to the new one. That is, as
opposed to having the same bits of the old VMA being mapped in the new
one, the new VMA will have an empty page table. This would slightly
change how userfaultfd's behavior on the new VMA, but that seems fine
to me.

- James

> --
> Mike Kravetz