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

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Message-ID: <e93f3fc9-00fd-5404-83f9-136b372e4867@redhat.com>
Date:   Fri, 17 Dec 2021 21:55:33 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 17.12.21 21:51, Linus Torvalds wrote:
> 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.

And I explained how it can not increment. And the only way is via
fork(), which cannot run concurrently.

> 
> 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?

If we have a shared anonymous page we cannot have GUP references, not
even R/O ones. Because GUP would have unshared and copied the page,
resulting in a R/O mapped anonymous page.

What am I missing?


-- 
Thanks,

David / dhildenb