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Date:   Thu, 7 Jan 2021 12:32:09 -0800
From:   Linus Torvalds <>
To:     Jason Gunthorpe <>
Cc:     Andrea Arcangeli <>,
        Linux-MM <>,
        Linux Kernel Mailing List <>,
        Yu Zhao <>, Andy Lutomirski <>,
        Peter Xu <>,
        Pavel Emelyanov <>,
        Mike Kravetz <>,
        Mike Rapoport <>,
        Minchan Kim <>,
        Will Deacon <>,
        Peter Zijlstra <>,
        Hugh Dickins <>,
        "Kirill A. Shutemov" <>,
        Matthew Wilcox <>,
        Oleg Nesterov <>, Jann Horn <>,
        Kees Cook <>,
        John Hubbard <>,
        Leon Romanovsky <>, Jan Kara <>,
        Kirill Tkhai <>
Subject: Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy

On Thu, Jan 7, 2021 at 12:25 PM Jason Gunthorpe <> 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_.


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