Re: [PATCH net-next 00/18] ipv6: Align nexthop behaviour with IPv4

[David Ahern <dsahern@gmail.com>]

On 1/7/18 3:45 AM, Ido Schimmel wrote:
> This set tries to eliminate some differences between IPv4's and IPv6's
> treatment of nexthops. These differences are most likely a side effect
> of IPv6's data structures (specifically 'rt6_info') that incorporate
> both the route and the nexthop and the late addition of ECMP support in
> commit 51ebd3181572 ("ipv6: add support of equal cost multipath
> (ECMP)").
> 
> IPv4 and IPv6 do not react the same to certain netdev events. For
> example, upon carrier change affected IPv4 nexthops are marked using the
> RTNH_F_LINKDOWN flag and the nexthop group is rebalanced accordingly.
> IPv6 on the other hand, does nothing which forces us to perform a
> carrier check during route lookup and dump. This makes it difficult to
> introduce features such as non-equal-cost multipath that are built on
> top of this set [1].
> 
> In addition, when a netdev is put administratively down IPv4 nexthops
> are marked using the RTNH_F_DEAD flag, whereas IPv6 simply flushes all
> the routes using these nexthops. To be consistent with IPv4, multipath
> routes should only be flushed when all nexthops in the group are
> considered dead.
> 
> The first 12 patches introduce non-functional changes that store the
> RTNH_F_DEAD and RTNH_F_LINKDOWN flags in IPv6 routes based on netdev
> events, in a similar fashion to IPv4. This allows us to remove the
> carrier check performed during route lookup and dump.
> 
> The next three patches make sure we only flush a multipath route when
> all of its nexthops are dead.
> 
> Last three patches add test cases for IPv4/IPv6 FIB. These verify that
> both address families react similarly to netdev events.
> 
> Finally, this series also serves as a good first step towards David
> Ahern's goal of treating nexthops as standalone objects [2], as it makes
> the code more in line with IPv4 where the nexthop and the nexthop group
> are separate objects from the route itself.
> 
> 1. https://github.com/idosch/linux/tree/ipv6-nexthops
> 2. http://vger.kernel.org/netconf2017_files/nexthop-objects.pdf
> 

Thanks for working on this - and creating the test cases.

One of many follow on changes that would be beneficial is to remove the
idev dereference in the hot path to check the
ignore_routes_with_linkdown sysctl.