diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index 4ea10db65..5539bb6f8 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -163,6 +163,12 @@ field was not zero.
 .SS UFFDIO_REGISTER
 (Since Linux 4.3.)
 Register a memory address range with the userfaultfd object.
+The pages in the range must be "compatible".
+In the current implementation,
+.\" According to Mike Rapoport, this will change in Linux 4.11.
+only private anonymous ranges are compatible for registering with
+.BR UFFDIO_REGISTER .
+
 The
 .I argp
 argument is a pointer to a