userfaultfd: hugetlbfs: UFFD_FEATURE_MISSING_HUGETLBFS

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

Link: http://lkml.kernel.org/r/20161216144821.5183-27-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>

diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index a3828a9bc16e67aa2b2919c85d393ffadce92e6a..7293321abdfb0117e769c14eb48da3189ef8ff41 100644 (file)
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -18,9 +18,10 @@
  * means the userland is reading).
  */
 #define UFFD_API ((__u64)0xAA)
-#define UFFD_API_FEATURES (UFFD_FEATURE_EVENT_FORK |       \
-                          UFFD_FEATURE_EVENT_REMAP |       \
-                          UFFD_FEATURE_EVENT_MADVDONTNEED)
+#define UFFD_API_FEATURES (UFFD_FEATURE_EVENT_FORK |           \
+                          UFFD_FEATURE_EVENT_REMAP |           \
+                          UFFD_FEATURE_EVENT_MADVDONTNEED |    \
+                          UFFD_FEATURE_MISSING_HUGETLBFS)
 #define UFFD_API_IOCTLS                                \
        ((__u64)1 << _UFFDIO_REGISTER |         \
         (__u64)1 << _UFFDIO_UNREGISTER |       \
@@ -125,11 +126,32 @@ struct uffdio_api {
         * Note: UFFD_EVENT_PAGEFAULT and UFFD_PAGEFAULT_FLAG_WRITE
         * are to be considered implicitly always enabled in all kernels as
         * long as the uffdio_api.api requested matches UFFD_API.
+        *
+        * UFFD_FEATURE_MISSING_HUGETLBFS means an UFFDIO_REGISTER
+        * with UFFDIO_REGISTER_MODE_MISSING mode will succeed on
+        * hugetlbfs virtual memory ranges. Adding or not adding
+        * UFFD_FEATURE_MISSING_HUGETLBFS to uffdio_api.features has
+        * no real functional effect after UFFDIO_API returns, but
+        * it's only useful for an initial feature set probe at
+        * UFFDIO_API time. There are two ways to use it:
+        *
+        * 1) by adding UFFD_FEATURE_MISSING_HUGETLBFS to the
+        *    uffdio_api.features before calling UFFDIO_API, an error
+        *    will be returned by UFFDIO_API on a kernel without
+        *    hugetlbfs missing support
+        *
+        * 2) the UFFD_FEATURE_MISSING_HUGETLBFS can not be added in
+        *    uffdio_api.features and instead it will be set by the
+        *    kernel in the uffdio_api.features if the kernel supports
+        *    it, so userland can later check if the feature flag is
+        *    present in uffdio_api.features after UFFDIO_API
+        *    succeeded.
         */
 #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)
        __u64 features;
 
        __u64 ioctls;