linux-kernel - Re: [PATCH] Mark timer_stats as incompatible with multiple pid namespaces

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Message-ID: <m1lk7yqn9s.fsf@ebiederm.dsl.xmission.com>
Date:	Thu, 13 Dec 2007 14:48:31 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linux Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Mark timer_stats as incompatible with multiple pid namespaces

Ingo Molnar <mingo@...e.hu> writes:

> * Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
>> >> Well struct pid * works in that case if you grab the reference to 
>> >> it.
>> >
>> > but the display of the stats might happen much later. The point of 
>> > this API is to save pid+comm, which gives users a good idea about 
>> > what caused the events in the past - without having to pin any 
>> > resource of that task.
>> 
>> Likewise struct pid is designed not to be a problem if pinned. It is a 
>> little heavier then it used to be with the addition of pid namespace 
>> support but not much.  And if it is to heavy struct pid needs to be 
>> fixed.
>> 
>> Holding the struct pid very much does not pin the task struct, and it 
>> shouldn't pin any other resources.  I agree 64bytes or so is a bit 
>> more to pin then 4 bytes but it really isn't a lot.
>
> yeah, and i have no conceptual objections - i just wanted to outline the 
> thinking behind /proc/timer_stats.

Sure.  Appreciated.  Outlining the thinking in the other direction
struct pid is supposed to be the pid representation in the kernel.
All of the pid_t stuff really should be pushed as close to the
kernel/user boundary as possible.  The closer I get to that ideal
the more cases I find that we need to handle like /proc/timer_stats
and the closer we get to having a complete pid namespace. <pant pant pant>

A struct pid is also now all you pin (besides the inevitable file,
dentry, inode trio) when you open a directory in /proc.  So the
task can be freed.  Which removed some low-mem exhaustion scenarios
when we introduced it.

Eric
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