[PATCH v10 2/6] userfaultfd: update documentation to describe UFFD_FEATURE_WP_ASYNC

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Explain the difference created by UFFD_FEATURE_WP_ASYNC to the write
protection (UFFDIO_WRITEPROTECT_MODE_WP) mode.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---
 Documentation/admin-guide/mm/userfaultfd.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 83f31919ebb3..4747e7bd5b26 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -221,6 +221,13 @@ former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter
 you still need to supply a page when ``UFFDIO_REGISTER_MODE_MISSING`` was
 used.
 
+If ``UFFD_FEATURE_WP_ASYNC`` is set while calling ``UFFDIO_API`` ioctl, the
+behaviour of ``UFFDIO_WRITEPROTECT_MODE_WP`` changes such that faults for
+anon and shmem are resolved automatically by the kernel instead of sending
+the message to the userfaultfd. The hugetlb isn't supported. The ``pagemap``
+file can be read to find which pages have ``PM_UFFD_WP`` flag set which
+means they are write-protected.
+
 QEMU/KVM
 ========
 
-- 
2.30.2