linux-kernel - Re: [RFC PATCH v2 25/47] hugetlb: add HGM support for copy_hugetlb_page

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Date:   Wed, 30 Nov 2022 18:18:43 -0500
From:   James Houghton <jthoughton@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        "Zach O'Keefe" <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 25/47] hugetlb: add HGM support for copy_hugetlb_page_range

On Wed, Nov 30, 2022 at 4:32 PM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Oct 21, 2022 at 04:36:41PM +0000, 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.
> >
> > A page's reference count is incremented for *each* portion of it that is
> > mapped in the page table. For example, if you have a PMD-mapped 1G page,
> > the reference count and mapcount will be incremented by 512.
> >
> > Signed-off-by: James Houghton <jthoughton@...gle.com>
>
> I have a feeling that this path is not triggered.  See:
>
> bcd51a3c679d ("hugetlb: lazy page table copies in fork()", 2022-07-17)
>
> It might be helpful to have it when exploring private mapping support of
> hgm on page poison in the future.  But the thing is if we want this to be
> accepted we still need a way to test it. I just don't see how to test this
> without the private support being there..

We can trigger this behavior by registering the VMA with
uffd-writeprotect. I didn't include any self-tests for this though;
I'll make sure to actually test this path in v1.

- James

>
> --
> Peter Xu
>