HMM mirror is a device driver helpers to mirror range of virtual address.
It means that the process jobs running on the device can access the same
virtual address as the CPU threads of that process.  This patch adds
support for mirroring mapping of file that are on a DAX block device (ie
range of virtual address that is an mmap of a file in a filesystem on a
DAX block device).  There is no reason to not support such case when
mirroring virtual address on a device.

Note that unlike GUP code we do not take page reference hence when we
back-off we have nothing to undo.

[jglisse@redhat.com: move THP and hugetlbfs code path behind #if KCONFIG]
  Link: http://lkml.kernel.org/r/20190422163741.13029-1-jglisse@redhat.com
Link: http://lkml.kernel.org/r/20190403193318.16478-10-jglisse@redhat.comSigned-off-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>