Re: [PATCH 25/33] userfaultfd: shmem: add userfaultfd hook for shared memory faults

[Andrea Arcangeli <aarcange@redhat.com>]

Hello,

I found a minor issue with the non cooperative testcase, sometime an
userfault would trigger in between UFFD_EVENT_MADVDONTNEED and
UFFDIO_UNREGISTER:

		case UFFD_EVENT_MADVDONTNEED:
			uffd_reg.range.start = msg.arg.madv_dn.start;
			uffd_reg.range.len = msg.arg.madv_dn.end -
				msg.arg.madv_dn.start;
			if (ioctl(uffd, UFFDIO_UNREGISTER, &uffd_reg.range))

It always triggered at the nr == 0:

	for (nr = 0; nr < nr_pages; nr++) {
		if (my_bcmp(area_dst + nr * page_size, zeropage, page_size))

The userfault still pending after UFFDIO_UNREGISTER returned, lead to
poll() getting a UFFD_EVENT_PAGEFAULT and trying to do a UFFDIO_COPY
into the unregistered range, which gracefully results in -EINVAL.

So this could be all handled in userland, by storing the MADV_DONTNEED
range and calling UFFDIO_WAKE instead of UFFDIO_COPY... but I think
it's more reliable to fix it into the kernel.

If a pending userfault happens before UFFDIO_UNREGISTER it'll just
behave like if it happened after.

I also noticed the order of uffd notification of MADV_DONTNEED and the
pagetable zap was wrong, we've to notify userland first so it won't
risk to call UFFDIO_COPY while the process runs zap_page_range.

With the two patches appended below the -EINVAL error out of
UFFDIO_COPY is gone.