From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 5/20] Introduce struct upid
Date: Tue, 14 Aug 2007 13:05:58 -0700
Message-ID: <20070814130558.4b5e5e3f.akpm@linux-foundation.org>
References: <46BC506F.mailGJ711PNHE@openvz.org>
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To: xemul@openvz.org
Cc: devel@openvz.org, linux-kernel@vger.kernel.org, containers@lists.osdl.org, oleg@tv-sign.ru, sukadev@us.ibm.com
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On Fri, 10 Aug 2007 15:47:59 +0400
xemul@openvz.org wrote:

>  struct pid
>  {
>  	atomic_t count;
> @@ -50,6 +50,8 @@ struct pid
>  	/* lists of tasks that use this pid */
>  	struct hlist_head tasks[PIDTYPE_MAX];
>  	struct rcu_head rcu;
> +	int level;
> +	struct upid numbers[1];

You can make this have size [0] now.  It's a gcc extension and
is used elsewhere in the kernel.