Merge branch 'netns-scalability'

Nicolas Dichtel says:

====================
netns: ease netlink use with a lot of netns

This idea was informally discussed in Ottawa / netdev0.1. The goal is to
ease the use/scalability of netns, from a userland point of view.
Today, users need to open one netlink socket per family and per netns.
Thus, when the number of netns inscreases (for example 5K or more), the
number of sockets needed to manage them grows a lot.

The goal of this series is to be able to monitor netlink events, for a
specified family, for a set of netns, with only one netlink socket. For
this purpose, a netlink socket option is added: NETLINK_LISTEN_ALL_NSID.
When this option is set on a netlink socket, this socket will receive
netlink notifications from all netns that have a nsid assigned into the
netns where the socket has been opened.
The nsid is sent to userland via an anscillary data.

Here is an example with a patched iproute2. vxlan10 is created in the
current netns (netns0, nsid 0) and then moved to another netns (netns1,
nsid 1):

$ ip netns exec netns0 ip monitor all-nsid label
[nsid 0][NSID]nsid 1 (iproute2 netns name: netns1)
[nsid 0][NEIGH]??? lladdr 00:00:00:00:00:00 REACHABLE,PERMANENT
[nsid 0][LINK]5: vxlan10@NONE: <BROADCAST,MULTICAST> mtu 1450 qdisc noop state DOWN group default
    link/ether 92:33:17:e6:e7:1d brd ff:ff:ff:ff:ff:ff
[nsid 0][LINK]Deleted 5: vxlan10@NONE: <BROADCAST,MULTICAST> mtu 1450 qdisc noop state DOWN group default
    link/ether 92:33:17:e6:e7:1d brd ff:ff:ff:ff:ff:ff
[nsid 1][NSID]nsid 0 (iproute2 netns name: netns0)
[nsid 1][LINK]5: vxlan10@NONE: <BROADCAST,MULTICAST> mtu 1450 qdisc noop state DOWN group default
    link/ether 92:33:17:e6:e7:1d brd ff:ff:ff:ff:ff:ff link-netnsid 0
[nsid 1][ADDR]5: vxlan10    inet 192.168.0.249/24 brd 192.168.0.255 scope global vxlan10
       valid_lft forever preferred_lft forever
[nsid 1][ROUTE]local 192.168.0.249 dev vxlan10  table local  proto kernel  scope host  src 192.168.0.249
[nsid 1][ROUTE]ff00::/8 dev vxlan10  table local  metric 256  pref medium
[nsid 1][ROUTE]2001:123::/64 dev vxlan10  proto kernel  metric 256  pref medium
[nsid 1][LINK]5: vxlan10@NONE: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UNKNOWN group default
    link/ether 92:33:17:e6:e7:1d brd ff:ff:ff:ff:ff:ff link-netnsid 0
[nsid 1][ROUTE]broadcast 192.168.0.255 dev vxlan10  table local  proto kernel  scope link  src 192.168.0.249
[nsid 1][ROUTE]192.168.0.0/24 dev vxlan10  proto kernel  scope link  src 192.168.0.249
[nsid 1][ROUTE]broadcast 192.168.0.0 dev vxlan10  table local  proto kernel  scope link  src 192.168.0.249
[nsid 1][ROUTE]fe80::/64 dev vxlan10  proto kernel  metric 256  pref medium
====================

Signed-off-by: David S. Miller <davem@davemloft.net>