[Lxc-users] mapping host PID <-> container PID

[Greg Kurz]

On Thu, 2011-04-28 at 09:41 -0500, Serge Hallyn wrote:
> Quoting Ulli Horlacher (framstag at rus.uni-stuttgart.de):
> > 
> > Is there a way to get the corresponding host PID for a container PID?
> > 
> > For example: inside the the container the process "init" has always PID 1.
> > But what PID has this process in the host process table?
> > 
> > ps aux | grep ... is not what I am looking for, I want more robust solution.
> 
> There is nothing that gives you a 100% guaranteed correct race-free
> correspondence right now.  You can look under /proc/<pid>/root/proc/ to
> see the pids valid in the container, and you can relate output of
> lxc-ps --forest to ps --forest output.  But nothing under /proc that I
> know of tells you "this task is the same as that task".  You can't
> even look at /proc/<pid> inode numbers since they are different
> filesystems for each proc mount.
> 
> It's tempting to say that we should put a per-task unique id under
> /proc/<pid> for each task.  However that would likely be nacked because
> it introduces a new namespace of its own.
> 

An alternative could be to expose the container pid
in /proc/<pid>/status. Could such a patch make it to mainline ?

--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -337,6 +337,12 @@ static void task_cpus_allowed(struct seq_file *m,
struct task_struct *task)
 	seq_putc(m, '\n');
 }
 
+static void task_vpid(struct seq_file *m, struct task_struct *task)
+{
+	struct pid_namespace *ns = task_active_pid_ns(task);
+	seq_printf(m, "Vpid:\t%d\n", ns ? task_pid_nr_ns(task, ns) : 0);
+}
+
 int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
 			struct pid *pid, struct task_struct *task)
 {
@@ -354,6 +360,7 @@ int proc_pid_status(struct seq_file *m, struct
pid_namespace *ns,
 	task_cpus_allowed(m, task);
 	cpuset_task_status_allowed(m, task);
 	task_context_switch_counts(m, task);
+	task_vpid(m, task);
 	return 0;
 }

Signed-off-by: Greg Kurz <gkurz at fr.ibm.com>