linux-kernel - Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG

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Message-ID: <CAHk-=wiR2Q5TQn_Vy10esOOshAego4wTCxgfDtVCxAw74hP5hg@mail.gmail.com>
Date:   Fri, 17 Dec 2021 13:50:08 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)

On Fri, Dec 17, 2021 at 1:47 PM David Hildenbrand <david@...hat.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