netdev - Re: [RFC PATCH net-next v2 0/5] netns: allow to identify peer netns

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Date:	Thu, 02 Oct 2014 12:45:54 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Network Development <netdev@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [RFC PATCH net-next v2 0/5] netns: allow to identify peer netns

Andy Lutomirski <luto@...capital.net> writes:

> On Thu, Oct 2, 2014 at 12:20 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:
>>
>>> Le 29/09/2014 20:43, Eric W. Biederman a écrit :
>>>> Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:
>>>>
>>>>> Le 26/09/2014 20:57, Eric W. Biederman a écrit :
>>>>>> Andy Lutomirski <luto@...capital.net> writes:
>>>>>>
>>>>>>> On Fri, Sep 26, 2014 at 11:10 AM, Eric W. Biederman
>>>>>>> <ebiederm@...ssion.com> wrote:
>>>>>>>> I see two ways to go with this.
>>>>>>>>
>>>>>>>> - A per network namespace table to that you can store ids for ``peer''
>>>>>>>>     network namespaces.  The table would need to be populated manually by
>>>>>>>>     the likes of ip netns add.
>>>>>>>>
>>>>>>>>     That flips the order of assignment and makes this idea solid.
>>>>> I have a preference for this solution, because it allows to have a full
>>>>> broadcast messages. When you have a lot of network interfaces (> 10k),
>>>>> it saves a lot of time to avoid another request to get all informations.
>>>>
>>>> My practical question is how often does it happen that we care?
>>> In fact, I don't think that scenarii with a lot of netns have a full mesh of
>>> x-netns interfaces. It will be more one "link" netns with the physical
>>> interface and all other with one interface with the link part in this "link"
>>> netns. Hence, only one nsid is needing in each netns.
>>
>> I will buy that a full mesh is unlikely.
>>
>> For people doing simulations anything physical has a limited number of
>> links.
>>
>> For people wanting all to all connectivity setting up an internal
>> macvlan (or the equivalent) is likely much simpler and more efficient
>> that a full mesh.
>>
>> So the question in my mind is how do we create these identifiers at need
>> (when we create the cross network namespace links) instead of at network
>> namespace creation time.  I don't see an answer to that in your patches,
>> and perhaps it obvious.
>>
>
> I wonder whether part of the problem is that we're thinking about
> scoping wrong.  What if we made the hierarchy more explicit?
>
> For example, we could give each netns an admin-assigned identifier
> (e.g. a 64-bit number, maybe required to be unique, maybe not)
> relative to its containing userns.  Then we could come up with a way
> to identify user namespaces (i.e. inode number relative to containing
> user ns, if that's well-defined).

If as suggested we only assign ids when a tunnel (or equivalent) is
created between two network namespaces the space cost is a non-issue.
The ids become at worst a constant factor addition to the cost of the
tunnel.

To keep things simple we may want to assign a free id (if one does not
exist) when we connect a tunnel to a network namespace.

> From user code's perspective, netnses that are in the requester's
> userns or its descendents are identified by a path through a (possibly
> zero-length) sequence of userns ids followed by a netns id.  Netnses
> outside the requester's userns hierarchy cannot be named at all.
>
> Would this make sense? 

Nope.  What happens if I migrate 2 of the 4 network namespaces in a user
namespace?  The migration potentially fails.  Application migration does
not require user namespace migration.

Eric
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