Messages in this thread | | | Date | Fri, 11 Nov 2011 16:25:32 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/3] pids: Make it possible to clone tasks with given pids |
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On 11/11, Pavel Emelyanov wrote: > > >> Unless: you are using CLONE_NEWPID along with CLONE_CHILD_USEPIDS and > >> this child_tidptr array has only one pid (before zero pid). > > > > And, if you do clone(CLONE_NEWPID | CLONE_CHILD_USEPIDS), then > > new_ns->child_reaper == NULL (unless you pass "1" in child_tidptr[]) ? > > > >> So, could you please explain what I have missed? > > > > please ;) I guess I misread this patch completely. Help! > > This is how I plan to use this functionality. > > When creating an init of a container being restored I call > > pids[0] = 1; > pids[1] = 0; > > clone(CLONE_NEWPID | CLONE_CHILD_USEPIDS, &pids)
Yep, this is clear. In this case everything works because the pid_ns has no pids (and thus ->last_pid == 0).
But. Let me repeat the question, what if you do the same with pids[0] = 2 /* anything != 1 */ ? In this case we create the new pid_ns, but its ->child_reaper is NULL. Unless I missed something.
> Then this created "init" task will have to read pids > from image files and call > > pids[0] = <pid> > pids[1] = 0 > > clone(CLONE_CHILD_USEPIDS, &pids); > > one by one. At this point the last_pid is still 0
Yes, understood. set_pidmap() bypasses the last_pid logic.
Clever hack^Wtrick ;)
May be this deserves a comment above "if (pid_ns->last_pid != 0)", and perhaps it would be more clean to do this check before anything else.
Hmm. It seems, we can make a simpler patch to achieve the (roughly) same effect. Without touching copy_process/alloc_pid paths. What if we simply add PR_SET_LAST_PID? (or something else).
In this case the new init (created normally) read the pids from image file and does prcrl(PR_SET_LAST_PID, pid-1) before the next fork.
What do you think?
Oleg.
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