sysctl: add the kernel.ns_last_pid control

The sysctl works on the current task's pid namespace, getting and setting
its last_pid field.

Writing is allowed for CAP_SYS_ADMIN-capable tasks thus making it possible
to create a task with desired pid value.  This ability is required badly
for the checkpoint/restore in userspace.

This approach suits all the parties for now.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index 6d8cd8b2c30d..8c20fbd8b42d 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -415,6 +415,14 @@ PIDs of value pid_max or larger are not allocated.
 
 ==============================================================
 
+ns_last_pid:
+
+The last pid allocated in the current (the one task using this sysctl
+lives in) pid namespace. When selecting a pid for a next task on fork
+kernel tries to allocate a number starting from this one.
+
+==============================================================
+
 powersave-nap: (PPC only)
 
 If set, Linux-PPC will use the 'nap' mode of powersaving,