userns; Document per user per user namespace limits.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Documentation/sysctl/README

@@ -69,6 +69,7 @@ proc/		<empty>
 sunrpc/		SUN Remote Procedure Call (NFS)
 vm/		memory management tuning
 		buffer and cache management
+user/		Per user per user namespace limits
 
 These are the subdirs I have on my system. There might be more
 or other subdirs in another setup. If you see another dir, I'd

Documentation/sysctl/user.txt

@@ -0,0 +1,66 @@
+Documentation for /proc/sys/user/*	kernel version 4.9.0
+	(c) 2016		Eric Biederman <ebiederm@xmission.com>
+
+==============================================================
+
+This file contains the documetation for the sysctl files in
+/proc/sys/user.
+
+The files in this directory can be used to override the default
+limits on the number of namespaces and other objects that have
+per user per user namespace limits.
+
+The primary purpose of these limits is to stop programs that
+malfunction and attempt to create a ridiculous number of objects,
+before the malfunction becomes a system wide problem.  It is the
+intention that the defaults of these limits are set high enough that
+no program in normal operation should run into these limits.
+
+The creation of per user per user namespace objects are charged to
+the user in the user namespace who created the object and
+verified to be below the per user limit in that user namespace.
+
+The creation of objects is also charged to all of the users
+who created user namespaces the creation of the object happens
+in (user namespaces can be nested) and verified to be below the per user
+limits in the user namespaces of those users.
+
+This recursive counting of created objects ensures that creating a
+user namespace does not allow a user to escape their current limits.
+
+Currently, these files are in /proc/sys/user:
+
+- max_cgroup_namespaces
+
+  The maximum number of cgroup namespaces that any user in the current
+  user namespace may create.
+
+- max_ipc_namespaces
+
+  The maximum number of ipc namespaces that any user in the current
+  user namespace may create.
+
+- max_mnt_namespaces
+
+  The maximum number of mount namespaces that any user in the current
+  user namespace may create.
+
+- max_net_namespaces
+
+  The maximum number of network namespaces that any user in the
+  current user namespace may create.
+
+- max_pid_namespaces
+
+  The maximum number of pid namespaces that any user in the current
+  user namespace may create.
+
+- max_user_namespaces
+
+  The maximum number of user namespaces that any user in the current
+  user namespace may create.
+
+- max_uts_namespaces
+
+  The maximum number of user namespaces that any user in the current
+  user namespace may create.