RE: [RFC]Pid conversion between pid namespace

[chenhanxiao@xxxxxxxxxxxxxx]

Hi,

Let me summarize our discussions of ID conversion by pros/cons: 

A) make new system call for translation	
    A-1) systemcall(ID, NS1, NS2) into (ID).
    pros:
        - has a reference ns(NS2)
          We could get any lower level ID directly.
		 
    cons:
        - lack of hierarchy information. 
          CRIU need hierarchy info for checkpoint/restore in nested containers.
        - not easy for debug. 
          And a lot of tools/libs need be modified.

    A-2) syscall pid_t getnspid(pid_t query_pid, pid_t observer_pid)
    pros:
        - ns procfs free, easy to use.
        We could get rid of mounted ns procfs.

    cons:
        - may find multiple results in nested ns.
          We wished the new API could tell us the exact answer.
          But if getnspid return more than one results will bring trouble to admins,
          they had to make another decision.
          Or we marked the deepest level for translation as prerequisite.

        -based on current pidns, no reference ns.

B) make/change proc file/directories
	B-1) expand /proc/pid/status
	pros:
        - easy to use and to debug
        - already had existed interface in kernel
        
	cons:
        - based on current ns
          for middle level, we had to make another decision.
        - do not have hierarchy info.

	B-2) /proc/<pidX>/ns/proc/ which would contain everything
	pros:
        - have enough info from /proc in container

	cons:
        - Requirements unclear.
          We need more discussion to decide which items should not be exposed.
        - do not have hierarchy info.


How about do these things in two steps: 

C)  1. expose all sets of pid, pgid, sid and tgid
via expanded /proc/PID/status
      We could get translated IDs from container like:
    NStgid:	16465 	5 	1 
    NSpid:	16465 	5 	1 
    NSpgid:	16465 	5 	1 
    NSsid:	16423 	1 	0
    (a set of IDs with 3 level of ns)

    2. add hierarchy info under /proc
      We lacked of method of getting hierarchy info, which is useful.
      Then we could know the relationship of ns.
      How about adding a new proc file just under /proc
      to show the hierarchy like readlink did:
	  pid:[4026531836]-> [4026532390] -> [4026532484]
      pid:[4026531836]-> [4026532491]
      (A 3 level pid and 2 level pid_

Any comments would be appreciated.

Thanks,
- Chen

> -----Original Message-----
> Subject: [RFC]Pid conversion between pid namespace
> 
> 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
> 
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