Re: [RFC] [PATCH 00/13] Introduce task_pid api

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Pavel Machek (pavel@ucw.cz):
> Hi!
> 
> > > Hmm... it is hard to judge a patch without context. Anyway, can't we
> > > get process snasphot/resume without virtualizing pids? Could we switch
> > > to 128-bits so that pids are never reused or something like that?
> > 
> > That might work fine for a managed cluster, but it wouldn't be a good
> > fit if you ever wanted to support something like a laptop in
> > disconnected operation, or if you ever want to restore the same snapshot
> > more than once.  There may also be some practical userspace issues
> > making pids that large.
> > 
> > I also hate bloating types and making them sparse just for the hell of
> > it.  It is seriously demoralizing to do a ps and see
> > 7011827128432950176177290 staring back at you. :)
> 
> Well, doing cat /var/something/foo.pid, and seeing pid of unrelated process
> is wrong, too... especially if you try to kill it....

Good point.  However the foo.pid scheme is incompatible with
checkpoint/restart and migration regardless.

	a. what good is trying to kill something using such a file if
	the process is checkpointed+killed, to be restarted later?
	b. it is expected that any files used by a checkpointable
	processes exist on a network fs, so that the fd can be moved.
	What good is foo.pid if it's on a network filesystem?

So if you wanted to checkpoint and restart/migrate a process with a
foo.pid type of file, you might need to start it with a private
tmpfs in a private namespace.  That part is trivial to do as part
of the management tools, though checkpointing a whole tmpfs per process
could be unfortunate.

-serge
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