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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 17 Dec 2021 13:50:08 -0800
Message-ID: <CAHk-=wiR2Q5TQn_Vy10esOOshAego4wTCxgfDtVCxAw74hP5hg@mail.gmail.com>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
To:     David Hildenbrand <david@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
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On Fri, Dec 17, 2021 at 1:47 PM David Hildenbrand <david@redhat.com> wrote:
>
> For now I have not heard a compelling argument why the mapcount is
> dubious, I repeat:
>
> * mapcount can only increase due to fork()
> * mapcount can decrease due to unmap / zap
>
> We can protect from the transtition == 1 -> >1 using the mmap_lock.
>
> For COW the mapcount is the only thing that matters *if we take GUP* out
> of the equation. And that's exactly what we

What do you have against just doing what we already do in other parts,
that a/b thing?

Which avoids the whole mmap_sem issue. That was a big issue for the
rdma people, afaik.

                Linus