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Date:   Fri, 8 Jan 2021 10:38:09 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, Yu Zhao <yuzhao@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Minchan Kim <minchan@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        John Hubbard <jhubbard@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy

On Fri, Jan 8, 2021 at 10:31 AM Andy Lutomirski <luto@...nel.org> wrote:
>
> Can we just remove vmsplice() support?  We could make it do a normal
> copy, thereby getting rid of a fair amount of nastiness and potential
> attacks.  Even ignoring issues relating to the length of time that the
> vmsplice reference is alive, we also have whatever problems could be
> caused by a malicious or misguided user vmsplice()ing some memory and
> then modifying it.

Well, that "misguided user" is kind of the point, originally. That's
what zero-copying is all about.

But we could certainly remove it in favor of copying, because
zero-copy has seldom really been a huge advantage in practice outside
of benchmarks.

That said, I continue to not buy into Andrea's argument that
page_count() is wrong.

Instead, the argument is:

 (1) COW can never happen "too much": the definition of a private
mapping is that you have your own copy of the data.

 (2) the one counter case I feel is valid is page pinning when used
for a special "pseudo-shared memory" thing and that's basically what
FOLL_GUP does.

So _regardless_ of any vmsplice issues, I actually think that those
two basic rules should be our guiding principle.

And the corollary to (2) is that COW must absolutely NEVER re-use too
little. And that _was_ the bug with vmsplice, in that it allowed
re-use that it shouldn't have.

           Linus

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