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

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Message-ID: <Y4fL1SjtFif4/REe@x1n>
Date:   Wed, 30 Nov 2022 16:32:05 -0500
From:   Peter Xu <peterx@...hat.com>
To:     James Houghton <jthoughton@...gle.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 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..

-- 
Peter Xu