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

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Message-ID: <CAHk-=wgD1Y_hV3eCRL55m+sGfmwic0wbq+0LV8youYKirHJTog@mail.gmail.com>
Date:   Fri, 17 Dec 2021 12:39:41 -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 12:36 PM Linus Torvalds
<torvalds@...ux-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