This is my preferred approach BY FAR.
Doing a <container,pid> approach is very natural, and avoids almost all
issues. At most, you might want to have a new system call (most naturally
just the one that is limited to the "init container" - it the one that we
boot up with) that can specify both container and pid explicitly, and see
all processes and access all processes. But all "normal" system calls
would only ever operate within their container.
The fact is, we want "containers" anyway for any virtualization thing, ie
vserver already adds them. And if we have containers, then it's very easy
("easyish") to split up the current static "pid_hash[]", "pidmap_array[]"
and "pidmap_lock", and make them per-container, and have a pointer to the
container for each "struct task_struct".
In fact this is almost what OpenVZ does for half a year, both containers
and VPIDs.
But it is very usefull to see process tree from host system. To be able
to use std tools to manage containers from host (i.e. ps, kill, top,
etc.). So it is much more convinient to have 2 pids. One globally
unique, and one for container.
After that, there wouldn't even be a lot else to do. The normal system
calls would just use their own container, and the (few) places that save
away pid's for later (ie things that use "kill_proc_info_as_uid()" and
"struct fown_struct" friends) would have to also squirrell away the
container, but then you should be pretty much done.
Of course, you'll have to do the system calls to _create_ the containers
in the first place, but that's at a higher level and involves much more
than just the pid-space (ie a container would normally have more than just
the uid mappings, it would have any network knowledge too etc - hostname,
perhaps list of network devices associated with that context etc etc)
Kirill
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