Document the new mnt_ns_id parameter to struct mnt_id_req and the
STATMOUNT_MNT_NS_ID flag.

req.mnt_ns_id can be used to query for a mount in a foreign mount
namespace.

STATMOUNT_MNT_NS_ID can be used to retreive the mnt_ns_id of the mount
being queried.

The mnt_ns_id parameter description is based on this commit message [1]
and STATMOUNT_MNT_NS_ID text is based on [2].

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=71aacb4c8c3d19da053363a5fe7538a8af082d56>
Link [2]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=09b31295f833031c88419550172703d45c5401e3>

Signed-off-by: Bhavik Sachdev <redacted>
---
 man/man2/statmount.2 | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/man/man2/statmount.2 b/man/man2/statmount.2
index cdc96da92..f5c4da122 100644
--- a/man/man2/statmount.2
+++ b/man/man2/statmount.2

@@ -23,9 +23,10 @@ .SH SYNOPSIS
 .P
 .EX
 .B struct mnt_id_req {
-.BR "    __u32  size;" "    /* sizeof(struct mnt_id_req) */"
-.BR "    __u64  mnt_id;" "  /* The mnt_id being queried */"
-.BR "    __u64  param;" "   /* An ORed combination of the STATMOUNT_ constants */"
+.BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
+.BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
+.BR "    __u64  param;" "       /* An ORed combination of the STATMOUNT_ constants */"
+.BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"
 .B };
 .P
 .B struct statmount {

@@ -47,6 +48,7 @@ .SH SYNOPSIS
 .B "    __u64  propagate_from;"
 .B "    __u32  mnt_root;"
 .B "    __u32  mnt_point;"
+.B "    __u64  mnt_ns_id;"
 .B "    char   str[];"
 .B };
 .EE

@@ -59,6 +61,9 @@ .SH SYNOPSIS
 .SH DESCRIPTION
 To access a mount's status,
 the caller must have CAP_SYS_ADMIN in the user namespace.
+In case of accessing a mount in a foreign mount namespace (specified via
+.IR req.mnt_ns_id ),
+the foreign mount namespace should be child of the current namespace.
 .P
 This function returns information about a mount,
 storing it in the buffer pointed to by

@@ -102,6 +107,7 @@ .SS The mnt_id_req structure
 STATMOUNT_MNT_ROOT	/* Want/got mnt_root  */
 STATMOUNT_MNT_POINT	/* Want/got mnt_point */
 STATMOUNT_FS_TYPE	/* Want/got fs_type */
+STATMOUNT_MNT_NS_ID	/* Want/got mnt_ns_id */
 .TE
 .in
 .P

@@ -130,6 +136,13 @@ .SS The mnt_id_req structure
 as one or more bits may,
 in the future,
 be used to specify an extension to the buffer.
+.P
+.I req.mnt_ns_id
+can be obtained from
+.B NS_GET_MNTNS_ID

+.BR ioctl (2)
+operation and is used to specify a foreign mount namespace in which to query
+.IR req.mnt_id .
 .SS The returned information
 The status information for the target mount is returned in the
 .I statmount

@@ -232,6 +245,9 @@ .SS The returned information
 relative to the current root (ie if you are in a
 .BR chroot ).
 It is a null-terminated string.
+.TP
+.I smbuf.mnt_ns_id