netdev - Re: [RFC][PATCH] ns: Syscalls for better namespace sharing control.

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Date:	Fri, 26 Feb 2010 14:16:43 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	Ben Greear <greearb@...delatech.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	containers@...ts.linux-foundation.org,
	Netfilter Development Mailinglist 
	<netfilter-devel@...r.kernel.org>,
	Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [RFC][PATCH] ns: Syscalls for better namespace sharing control.

Oren Laadan <orenl@...columbia.edu> writes:

> Eric W. Biederman wrote:
>> Pavel Emelyanov <xemul@...allels.com> writes:
>>
>>>>> Yet another set of per-namespace IDs along with CLONE_NEWXXX ones?
>>>>> I currently have a way to create all namespaces we have with one
>>>>> syscall. Why don't we have an ability to enter them all with one syscall?
>>>> The CLONE_NEWXXX series of bits has been an royal pain to work with,
>>>> and it appears to be unnecessary complications for no gain.
>>> That's the answer for the "Yet another set..." question.
>>> How about the "Why don't we have..." one?
>>
>> I am not certain which question you are asking:
>>
>> Why don't we have an ability to enter all namespaces with one syscall
>> invocation?
>
> That's how I understood the question, and I, too, wonder why not ?
>
> By the way, an alternative to using bitmap is to change the prototype
> of setns() to accept an array of FD's:
>
> 	int setns(int *fds, int nfds);
>
> So the process will atomically enter all the namespaces as specified
> by the FDs.

We could.  Mostly I implemented things in the simplest way possible.

Semantically I know of no reason why need to enter more than one namespace
at once, and I don't expect entering a namespace to be on anyone's fast
path so every last drop of performance was not crucial.

The only justification I can think of for more than one namespace at a
time is that because we have a synchronize_rcu() in the kernel we can
loop in the kernel much more quickly than we can loop in userspace.

Eric
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