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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 17 Dec 2021 12:39:41 -0800
Message-ID: <CAHk-=wgD1Y_hV3eCRL55m+sGfmwic0wbq+0LV8youYKirHJTog@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>,
        Michal Hocko <mhocko@kernel.org>,
        Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Linux-MM <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
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On Fri, Dec 17, 2021 at 12:36 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> >
> > 5. Take a R/O pin (RDMA, VFIO, ...)
> > -> refcount > 1
> >
> > 6. memset(mem, 0xff, pagesize);
> > -> Write fault -> COW
>
> I do not believe this is actually a bug.
>
> You asked for a R/O pin, and you got one.

If you want a shared pin that actually follows the changes of your
process around, then that is what you should have asked for.

At the time of such a shared pin, you can do what we already do:
re-use the page if it has a refcount of 1. Or do an early COW event
(feel free to avoid the "mark it writable and dirty").

But note: *refcount* of 1. Not "mapcount". Because mapcount would be
broken garbage.

                Linus