[RFC]Pid conversion between pid namespace

[chenhanxiao@xxxxxxxxxxxxxx]

Hi,

We had some discussions on how to carry out
pid conversion between pid namespace via:
syscall[1] and procfs[2].

Pavel suggested that a syscall like
(ID, NS1, NS2) into (ID).

Serge suggested that a syscall 
pid_t getnspid(pid_t query_pid, pid_t observer_pid).


Eric and Richard suggested a procfs solution is
more appropriate.

Oleg suggested that we should expand /proc/pid/status
to report this kind of information.

And Richard suggested adding a directory like
/proc/<pidX>/ns/proc/ which would contain everything
from /proc/<pidX inside the namespace>/.

As procfs provided a more user friendly interface,
how about expose all sets of tgid, pid, pgid, sid 
by expanding /proc/PID/status in procfs?
And we could also expose ns hierarchy under /proc,
which could be another reference.

Ex:
    init_pid_ns    ns1         ns2
t1  2
t2   `- 3          1 
t3       `- 4      `- 5        1

We could get in /proc/t3/status:
NSpid: 4 5 1
We knew that pid 1 in container is pid 4 in init ns.

And we could get ns hierarchy under /proc/ns_hierarchy like:
init_ns->ns1->ns2		(as the result of readlink)
         ->ns3
We knew that t3 in ns2, and its hierarchy.

How these ideas looks like?
Any comments would be appreciated.

Thanks,
- Chen


a) syscall
http://lwn.net/Articles/602987/

b) procfs
http://www.spinics.net/lists/kernel/msg1751688.html