userfaultfd: hugetlbfs: UFFD_FEATURE_MISSING_SHMEM

Userland developers asked to be notified immediately by the UFFDIO_API
ioctl if shmem missing mode is supported by userfaultfd in the running
kernel.  This avoids the need to run UFFDIO_REGISTER on a shmem virtual
memory range to find out.

Link: http://lkml.kernel.org/r/20161216144821.5183-38-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/uapi/linux/userfaultfd.h

@@ -21,7 +21,8 @@
 #define UFFD_API_FEATURES (UFFD_FEATURE_EVENT_FORK |		\
 			   UFFD_FEATURE_EVENT_REMAP |		\
 			   UFFD_FEATURE_EVENT_MADVDONTNEED |	\
-			   UFFD_FEATURE_MISSING_HUGETLBFS)
+			   UFFD_FEATURE_MISSING_HUGETLBFS |	\
+			   UFFD_FEATURE_MISSING_SHMEM)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -146,12 +147,17 @@ struct uffdio_api {
 	 *    it, so userland can later check if the feature flag is
 	 *    present in uffdio_api.features after UFFDIO_API
 	 *    succeeded.
+	 *
+	 * UFFD_FEATURE_MISSING_SHMEM works the same as
+	 * UFFD_FEATURE_MISSING_HUGETLBFS, but it applies to shmem
+	 * (i.e. tmpfs and other shmem based APIs).
 	 */
 #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
 #define UFFD_FEATURE_EVENT_FORK			(1<<1)
 #define UFFD_FEATURE_EVENT_REMAP		(1<<2)
 #define UFFD_FEATURE_EVENT_MADVDONTNEED		(1<<3)
 #define UFFD_FEATURE_MISSING_HUGETLBFS		(1<<4)
+#define UFFD_FEATURE_MISSING_SHMEM		(1<<5)
 	__u64 features;
 
 	__u64 ioctls;