Re: userfaultfd: unexpected behavior with MODE_MISSING | MODE_WP regions

[Andrea Arcangeli <aarcange@redhat.com>]

Hello Evgeny,

On Thu, Aug 11, 2016 at 04:51:30PM +0300, Evgeny Yakovlev wrote:
>   * 1. First fault is expected UFFD_PAGEFAULT_FLAG_WRITE set which we 
> resolve
>   * with zeropage

What if you resolve it with bzero(4096);UFFDIO_COPY? Does the problem
go away?

If the zeropage is mapped by UFFDIO_ZEROPAGE, there's no way to turn
that into a writable zeropage ever again because
userfaultfd_writeprotect is basically a no-vma-mangling mmap_sem-read
mprotect and it can't trigger faults. Instead a fault in do_wp_page is
required to get rid of the zeropage and copy it off.

If the problem goes away if you s/UFFDIO_ZEROPAGE/bzero(4096);
UFFDIO_COPY/ as I would expect, there would be two ways to solve it:

1) forbid UFFDIO_ZEROPAGE and not return the UFFDIO_ZEROPAGE ioctl in
   uffdio_register.ioctls, if UFFDIO_REGISTER is called with
   uffdio_register.mode = ...WP|..MISSING so userland is aware it
   can't use that.

2) teach UFFDIO_WRITEPROTECT not just to mangle pagetables but also
   trigger a write fault on any zeropage if it's called with
   uffdio_writeprotect.mode without UFFDIO_WRITEPROTECT_MODE_WP being
   set. This will require a bit more work to fix.

The latter would increase performance if not all zeropages needs to be
turned writable.

Feedback welcome on what solution would you prefer.

Thanks,
Andrea

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