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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 7 Jan 2021 12:32:09 -0800
Message-ID: <CAHk-=wjTuS9JB=Ms4WAMaOkGuLmvYwaf2W0JhXxNPdcv4NWZUA@mail.gmail.com>
Subject: Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Andrea Arcangeli <aarcange@redhat.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Yu Zhao <yuzhao@google.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>
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On Thu, Jan 7, 2021 at 12:25 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> Lots of places are relying on pin_user_pages long term pins of memory,
> and cannot be converted to notifiers.
>
> I don't think it is reasonable to just declare that insecure and
> requires privileges, it is a huge ABI break.

Also, I think GUP (and pin_user_pages() as a special case) is a lot
more important and more commonly used than UFFD.

Which is really why I think this needs to be fixed by just fixing UFFD
to take the write lock.

I think Andrea is blinded by his own love for UFFDIO: when I do a
debian codesearch for UFFDIO_WRITEPROTECT, all it finds is the kernel
and strace (and the qemu copies of the kernel headers).

Does the debian code search cover everything? Obviously not. But if
you cannot find A SINGLE USE of that thing in the Debian code search,
then that is sure a sign of _something_.

            Linus