linux-kernel - Re: [PATCH] [RFC] pidns: don't zap processes several times

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Message-ID: <20121007190118.GA16068@redhat.com>
Date:	Sun, 7 Oct 2012 21:01:18 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Vagin <avagin@...nvz.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH] [RFC] pidns: don't zap processes several times

On 10/07, Andrew Vagin wrote:
>
> I wrote a test program. It does clone(CLONE_NEWPID | CLONE_VM) and
> sleep(), a new task repeates the same actions. This program creates
> 4000 tasks. When I tried to kill all this processes, a system was
> inaccessible for some minutes.

So this creates 4000 nested namespaces? Not sure this really needs the
fix... The size of pid would be more than 4000 * sizeof(struct upid).

Perhaps we should MAX_PID_NS_LEVEL instead?

As for the patch, it looks correct at first glance. But,

> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -34,6 +34,7 @@ struct pid_namespace {
>  	kgid_t pid_gid;
>  	int hide_pid;
>  	int reboot;	/* group exit code if this pidns was rebooted */
> +	atomic_t zapped; /* non zero if all process were killed */
>  };

atomic_t buys nothing. In this case atomic_set/read doesn't differ
from plain STORE/LOAD.

> @@ -177,21 +177,31 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>  	 * 	  maintain a tasklist for each pid namespace.
>  	 *
>  	 */
> +
> +	if (atomic_read(&pid_ns->zapped))
> +		goto wait; /* All processes were already killed */
> +

OK, but if we try to speedup, then probably the main loop should
check ->zapped too and stop. Multiple reapers can start
zap_pid_ns_processes() at the same time.

So, probably,

>  	read_lock(&tasklist_lock);
>  	nr = next_pidmap(pid_ns, 1);
>  	while (nr > 0) {

should be "while (nr > 0 && !zapped)", and

>  		rcu_read_lock();
>
>  		task = pid_task(find_vpid(nr), PIDTYPE_PID);
> -		if (task && !__fatal_signal_pending(task))
> +		if (task && !__fatal_signal_pending(task)) {
> +			struct pid_namespace *ns;
> +
>  			send_sig_info(SIGKILL, SEND_SIG_FORCED, task);
> +			ns = task_active_pid_ns(task);
> +			if (unlikely(ns->child_reaper == task))
> +				atomic_set(&ns->zapped, 1);

This should be unconditional. Even if the task is not child_reaper,
we are going to kill the whole namespace. So I think

	if (task_active_pid_ns(task) != task_active_pid_ns(current))
		ns->zapped = 1;

except it should be optimized.



I am wondering if we can do for_each_pid_in_this_ns(pid) which skips
the pids from the sub-namespaces. Note that zap_pid_ns_processes()
doesn't really need to kill the tasks from sub-namespace, its init
will take care anyway. In this case we do not nee ns->zapped.
Probably not...

Oleg.

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