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

From: Nicolas Dichtel
Date: Mon Sep 29 2014 - 08:06:48 EST

Le 26/09/2014 20:57, Eric W. Biederman a Ãcrit :
Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:

On Fri, Sep 26, 2014 at 11:10 AM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
Nicolas Dichtel <nicolas.dichtel@xxxxxxxxx> writes:

The goal of this serie is to be able to multicast netlink messages with an
attribute that identify a peer netns.
This is needed by the userland to interpret some informations contained in
netlink messages (like IFLA_LINK value, but also some other attributes in case
of x-netns netdevice (see also and

I want say that the problem addressed by patch 3/5 of this series is a
fundamentally valid problem. We have network objects spanning network
namespaces and it would be very nice to be able to talk about them in
netlink, and file descriptors are too local and argubably too heavy
weight for netlink quires and especially for netlink broadcast messages.

Furthermore the concept of ineternal concept of peernet2id seems valid.

However what you do not address is a way for CRIU (aka process
migration) to be able to restore these ids after process migration.
Going farther it looks like you are actively breaking process migration
at this time, making this set of patches a no-go.
Ok, I will look more deeply into CRIU.

When adding a new form of namespace id CRIU patches are just about
as necessary as iproute patches.

Ids are stored in the parent user namespace. These ids are valid only inside
this user namespace. The user can retrieve these ids via a new netlink messages,
but only if peer netns are in the same user namespace.

That does not describe what you have actually implemented in the

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.

Unfortunately in the case of a fully referencing mesh of N network
namespaces such a mesh winds up taking O(N^2) space, which seems
Memory consumption vs performances ;-)
In fact, when you have a lot of netns, you already should have some memory
available (at least N lo interfaces + N interfaces (veth or a x-netns
interface)). I'm not convinced that this is really an obstacle.

- Add a netlink attribute that says this network element is in a peer
network namespace.

Add a unicast query message that let's you ask if the remote
end of a tunnel is in a network namespace specified by file

I personally lean towards the second version as it is fundamentally
simpler, and generally scales better, and the visibility controls are
the existing visibility controls. The only downside is it requires
a query after receiving a netlink broadcast message for the times that
we care.

The downside of that approach, and all the similar kcmp stuff, is that
it scales poorly for applications using it. This is probably not the
end of the world, but it's not ideal.

Agreed, the efficiency is not ideal and there is plenty of room for
optimization. We could certainly adopt some of kcmps ordering
infrastructure to make it suck less, or even potentially work out how
to return a file descriptor to the network namespace in question.

The key insight of my second proposal is that we can get out of the
broadcast message business, and only care about the remote namespace for
unicast messages. Putting the work in an infrequently used slow path
instead of a comparitively common path gives us much more freedom in
the implementation.
I think it's better to have a full netlink messages, instead a partial one.
There is already a lot of attributes added for each rtnl interface messages to
be sure to describe all parameters of these interfaces.
And if the user don't care about ids (user has not set any id with iproute2),
we can just add the same attribute with id 0 (let's say it's a reserved id) to
indicate that the link part of this interface is in another netns.

The great benefit of your first proposal is that the ids are set by the
userspace and thus it allows a high flexibility.

Would you accept a patch that implements this first solution?
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at