namespaces.7: List factors that may pin a namespace into existence

Various factors may pin a namespace into existence, even when it
has no member processes.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/namespaces.7

@@ -412,6 +412,38 @@ and
 Use of UTS namespaces requires a kernel that is configured with the
 .B CONFIG_UTS_NS
 option.
+.\"
+.SS Namespace lifetime
+Absent any other factors,
+a namespace is automatically torn down when the last process in
+the namespace terminates or leaves the namespace.
+However, there are a number of other factors that may pin
+a namespace into existence even though it has no member processes.
+These factors include the following:
+.IP * 3
+An open file descriptor or a bind mount exists for the corresponding
+.IR /proc/[pid]/ns/*
+file.
+.IP *
+The namespace is hierarchical (i.e., a PID or user namespace),
+and has a child namespace.
+.IP *
+It is a user namespace that owns one or more nonuser namespaces.
+.IP *
+It is a PID namespace,
+and there is a process that refers to the namespace via a
+.IR /proc/[pid]/ns/pid_for_children
+symbolic link.
+.IP *
+It is an IPC namespace, and a corresponding mount of an
+.I mqueue
+filesystem (see
+.BR mq_overview (7))
+refers to this namespace.
+.IP *
+It is an PID namespace, and a corresponding mount of a
+.BR proc (5)
+filesystem refers to this namespace.
 .SH EXAMPLE
 See
 .BR clone (2)