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    Subject[RFC][patch 00/21] PID Virtualization: Overview and Patches
    This patchset is a followup to the posting by Serge.
    http://marc.theaimsgroup.com/?l=linux-kernel&m=113200410620972&w=2

    In this patchset here, we are providing the pid virtualization mentioned
    in serge's posting.

    > I'm part of a project implementing checkpoint/restart processes.
    > After a process or group of processes is checkpointed, killed, and
    > restarted, the changing of pids could confuse them. There are many
    > other such issues, but we wanted to start with pids.
    >
    > This patchset introduces functions to access task->pid and ->tgid,
    > and updates ->pid accessors to use the functions. This is in
    > preparation for a subsequent patchset which will separate the kernel
    > and virtualized pidspaces. This will allow us to virtualize pids
    > from users' pov, so that, for instance, a checkpointed set of
    > processes could be restarted with particular pids. Even though their
    > kernel pids may already be in use by new processes, the checkpointed
    > processes can be started in a new user pidspace with their old
    > virtual pid. This also gives vserver a simpler way to fake vserver
    > init processes as pid 1. Note that this does not change the kernel's
    > internal idea of pids, only what users see.
    >
    > The first 12 patches change all locations which access ->pid and
    > ->tgid to use the inlined functions. The last patch actually
    > introduces task_pid() and task_tgid(), and renames ->pid and ->tgid
    > to __pid and __tgid to make sure any uncaught users error out.
    >
    > Does something like this, presumably after much working over, seem
    > mergeable?

    These patches build on top of serge's posted patches (if necessary
    we can repost them here).

    PID Virtualization is based on the concept of a container.
    The ultimate goal is to checkpoint/restart containers.

    The mechanism to start a container
    is to 'echo "container_name" > /proc/container' which creates a new
    container and associates the calling process with it. All subsequently
    forked tasks then belong to that container.
    There is a separate pid space associated with each container.
    Only processes/task belonging to the same container "see" each other.
    The exception is an implied default system container that has
    a global view.

    The following patches accomplish 3 things:
    1) identify the locations at the user/kernel boundary where pids and
    related ids ( pgrp, sessionids, .. ) need to be (de-)virtualized and
    call appropriate (de-)virtualization functions.
    2) provide the virtualization implementation in these functions.
    3) implement a container object and a simple /proc interface to create one
    4) provide a per container /proc/fs

    -- Hubertus Franke (frankeh@watson.ibm.com)
    -- Cedric Le Goater (clg@fr.ibm.com)
    -- Serge E Hallyn (serue@us.ibm.com)
    -- Dave Hansen (haveblue@us.ibm.com)
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