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

[Daniel Lezcano]

On 05/03/2011 05:36 PM, Greg Kurz wrote:
> 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>

I think we should propose this patch for mainline inclusion. The vpid 
does not give, by its own, enough information for the pid namespace. How 
can we rebuild a pid ns tree ? I guess we can look for the vpid 1 as the 
root node of the process tree no ?

Otherwise:
Acked-by: Daniel Lezcano <daniel.lezcano at free.fr>