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

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Message-ID: <CAHk-=wghsZByyzCqb5EbKzZtAbrFvQCViD+jK9HQL4viqUb6Ow@mail.gmail.com>
Date:   Fri, 17 Dec 2021 12:51:22 -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 12:45 PM David Hildenbrand <david@...hat.com> wrote:
>
> If a page is mapped exactly once, page_mapcount(page) == 1 and there is
> nothing to do.

Why?

You state that, but you stating that doesn't magically make it so.

What makes "mapcount==1" stable and special? Your "it's an
atomic_read()" argument is nonsense - it implies that the count can be
changing, but you will get _one_ answer.

What makes that one answer of a changing count special?

What if there are other references to that same page, gotten with
vmsplice(), and just about to be mapped into another address space?

This is the meat of my argument. You claim that "mapcount==1" is
special. I claim that you haven't explained why it would be. And I do
not believe it is.

                 Linus