[RFC]Pid conversion between pid namespace

[Yasunori Goto]

Chen-san,

I would like to recommend that you summarize pros/cons for all ideas so far.

For example,

---------
A) make new system call for transrate 

   A-1) systemcall(ID, NS1, NS2) into (ID).
     pros: 
        - foo
        - baa

     cons: 
        - hoge
        - hogehogehoge

   A-2) pid_t getnspid(pid_t query_pid, pid_t observer_pid) 
      (ditto)


B) make/change proc file/directories
   B-1) expand /proc/pid/status
       (ditto)
   
   B-2) /proc/<pidX>/ns/proc/ which would contain everything
         from /proc/<pidX inside the namespace>/.
       (ditto)


------

Please make clear what is the good/bad point of each opinion by the above,
  - Is it hard to keep compatiblity?
  - Is it hard to understand for administorator/programmer?
  - Is it difficult to show for "nested containers"?
  - Is userland tool necessary?
  - any other problems?

I hope it will be good discussion by the above.

Thanks,

> 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
> 

-- 
Yasunori Goto <y-goto at jp.fujitsu.com>