Re: [PATCH 0/2] page_count can't be used to decide when wp_page_copy

From: Linus Torvalds
Date: Thu Jan 07 2021 - 15:33:13 EST

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