[PATCH] Userfaultfd: Add description for UFFD_FEATURE_SIGBUS

[Prakash Sangappa <prakash.sangappa@oracle.com>]

Userfaultfd feature UFFD_FEATURE_SIGBUS was merged recently and should
be available in Linux 4.14 release. This patch is for the manpage
changes documenting this API.

Documents the following commit:

commit 2d6d6f5a09a96cc1fec7ed992b825e05f64cb50e
Author: Prakash Sangappa <prakash.sangappa@oracle.com>
Date: Wed Sep 6 16:23:39 2017 -0700

    mm: userfaultfd: add feature to request for a signal delivery

Signed-off-by: Prakash Sangappa <prakash.sangappa@oracle.com>
---
 man2/ioctl_userfaultfd.2 |  9 +++++++++
 man2/userfaultfd.2       | 17 +++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index 60fd29b..cfc65ae 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -196,6 +196,15 @@ with the
 flag set,
 .BR memfd_create (2),
 and so on.
+.TP
+.B UFFD_FEATURE_SIGBUS
+Since Linux 4.14, If this feature bit is set, no page-fault events(
+.B UFFD_EVENT_PAGEFAULT
+) will be delivered, instead a
+.B SIGBUS
+signal will be sent to the faulting process. Applications using this
+feature will not require the use of a userfaultfd monitor for handling
+page-fault events.
 .IP
 The returned
 .I ioctls
diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index 1741ee3..a033742 100644
--- a/man2/userfaultfd.2
+++ b/man2/userfaultfd.2
@@ -172,6 +172,23 @@ or
 .BR ioctl (2)
 operations to resolve the page fault.
 .PP
+Starting from Linux 4.14, if application sets
+.B UFFD_FEATURE_SIGBUS
+feature bit using
+.B UFFDIO_API
+.BR ioctl (2)
+, no page fault notification will be forwarded to
+the user-space, instead a
+.B SIGBUS
+signal is delivered to the faulting process. With this feature,
+userfaultfd can be used for robustness purpose to simply catch
+any access to areas within the registered address range that do not
+have pages allocated, without having to deal with page-fault events.
+No userfaultd monitor will be required for handling page faults. For
+example, this feature can be useful for applications that want to
+prevent the kernel from automatically allocating pages and filling
+holes in sparse files when the hole is accessed thru mapped address.
+.PP
 Details of the various
 .BR ioctl (2)
 operations can be found in
-- 
2.7.4

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