mm: always enable thp for dax mappings

mm: always enable thp for dax mappings

The madvise policy for transparent huge pages is meant to avoid unwanted
allocations of transparent huge pages.  It allows a policy of disabling
the extra memory pressure and effort to arrange for a huge page when it
is not needed.

DAX by definition never incurs this overhead since it is statically
allocated.  The policy choice makes even less sense for device-dax which
tries to guarantee a given tlb-fault size.  Specifically, the following
setting:

	echo never > /sys/kernel/mm/transparent_hugepage/enabled

...violates that guarantee and silently disables all device-dax
instances with a 2M or 1G alignment.  So, let's avoid that non-obvious
side effect by force enabling thp for dax mappings in all cases.

It is worth noting that the reason this uses vma_is_dax(), and the
resulting header include changes, is that previous attempts to add a
VM_DAX flag were NAKd.

Link: http://lkml.kernel.org/r/149739531127.20686.15813586620597484283.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>