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

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Date:   Tue, 12 Jul 2022 11:06:55 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     James Houghton <jthoughton@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...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 07/12/22 10:19, James Houghton wrote:
> 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.

Yes, for now.  We will not copy the page tables for shared mappings.
When adding support for private mapping, we will need to handle the
HGM case.

>                                                       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.

Right.  Since the 'mapping size information' is essentially carried in
the page tables, it will be lost if page tables are not copied.

Not sure if anyone would depend on that behavior.

Axel, this may also impact minor fault processing.  Any concerns?
Patch is sitting in Andrew's tree for next merge window.
-- 
Mike Kravetz