[PATCH v6 05/10] userfaultfd/shmem: advertise shmem minor fault support

[Axel Rasmussen]

Now that the feature is fully implemented (the faulting path hooks exist
so userspace is notified, and the ioctl to resolve such faults is
available), advertise this as a supported feature.

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
 Documentation/admin-guide/mm/userfaultfd.rst | 3 ++-
 fs/userfaultfd.c                             | 3 ++-
 include/uapi/linux/userfaultfd.h             | 7 ++++++-
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 3aa38e8b8361..6528036093e1 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -77,7 +77,8 @@ events, except page fault notifications, may be generated:
 
 - ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports
   ``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory
-  areas.
+  areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
+  support for shmem virtual memory areas.
 
 The userland application should set the feature flags it intends to use
 when invoking the ``UFFDIO_API`` ioctl, to request that those features be
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 468556fb04a9..9f3b8684cf3c 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1940,7 +1940,8 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 	/* report all available features and ioctls to userland */
 	uffdio_api.features = UFFD_API_FEATURES;
 #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
-	uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS;
+	uffdio_api.features &=
+		~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
 #endif
 	uffdio_api.ioctls = UFFD_API_IOCTLS;
 	ret = -EFAULT;
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index bafbeb1a2624..159a74e9564f 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -31,7 +31,8 @@
 			   UFFD_FEATURE_MISSING_SHMEM |		\
 			   UFFD_FEATURE_SIGBUS |		\
 			   UFFD_FEATURE_THREAD_ID |		\
-			   UFFD_FEATURE_MINOR_HUGETLBFS)
+			   UFFD_FEATURE_MINOR_HUGETLBFS |	\
+			   UFFD_FEATURE_MINOR_SHMEM)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -185,6 +186,9 @@ struct uffdio_api {
 	 * UFFD_FEATURE_MINOR_HUGETLBFS indicates that minor faults
 	 * can be intercepted (via REGISTER_MODE_MINOR) for
 	 * hugetlbfs-backed pages.
+	 *
+	 * UFFD_FEATURE_MINOR_SHMEM indicates the same support as
+	 * UFFD_FEATURE_MINOR_HUGETLBFS, but for shmem-backed pages instead.
 	 */
 #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
 #define UFFD_FEATURE_EVENT_FORK			(1<<1)
@@ -196,6 +200,7 @@ struct uffdio_api {
 #define UFFD_FEATURE_SIGBUS			(1<<7)
 #define UFFD_FEATURE_THREAD_ID			(1<<8)
 #define UFFD_FEATURE_MINOR_HUGETLBFS		(1<<9)
+#define UFFD_FEATURE_MINOR_SHMEM		(1<<10)
 	__u64 features;
 
 	__u64 ioctls;
-- 
2.31.1.527.g47e6f16901-goog