Re: [net-next] ipv6: fix routing cache overflow for raw sockets

From: Jonathan Maxwell
Date: Tue Dec 20 2022 - 16:56:25 EST


On Wed, Dec 21, 2022 at 2:10 AM David Ahern <dsahern@xxxxxxxxxx> wrote:
>
> On 12/20/22 5:35 AM, Paolo Abeni wrote:
> > On Mon, 2022-12-19 at 10:48 +1100, Jon Maxwell wrote:
> >> Sending Ipv6 packets in a loop via a raw socket triggers an issue where a
> >> route is cloned by ip6_rt_cache_alloc() for each packet sent. This quickly
> >> consumes the Ipv6 max_size threshold which defaults to 4096 resulting in
> >> these warnings:
> >>
> >> [1] 99.187805] dst_alloc: 7728 callbacks suppressed
> >> [2] Route cache is full: consider increasing sysctl net.ipv6.route.max_size.
> >> .
> >> .
> >> [300] Route cache is full: consider increasing sysctl net.ipv6.route.max_size.
> >
> > If I read correctly, the maximum number of dst that the raw socket can
> > use this way is limited by the number of packets it allows via the
> > sndbuf limit, right?
> >
> > Are other FLOWI_FLAG_KNOWN_NH users affected, too? e.g. nf_dup_ipv6,
> > ipvs, seg6?
> >
> > @DavidA: why do we need to create RTF_CACHE clones for KNOWN_NH flows?
> >
> > Thanks,
> >
> > Paolo
> >
>
> If I recall the details correctly: that sysctl limit was added back when
> ipv6 routes were managed as dst_entries and there was a desire to allow
> an admin to limit the memory consumed. At this point in time, IPv6 is
> more inline with IPv4 - a separate struct for fib entries from dst
> entries. That "Route cache is full" message is now out of date since
> this is dst_entries which have a gc mechanism.
>
> IPv4 does not limit the number of dst_entries that can be allocated
> (ip_rt_max_size is the sysctl variable behind the ipv4 version of
> max_size and it is a no-op). IPv6 can probably do the same here?
>

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index dbc224023977..701aba7feaf5 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -6470,7 +6470,7 @@ static int __net_init ip6_route_net_init(struct net *net)
#endif

net->ipv6.sysctl.flush_delay = 0;
- net->ipv6.sysctl.ip6_rt_max_size = 4096;
+ net->ipv6.sysctl.ip6_rt_max_size = INT_MAX;
net->ipv6.sysctl.ip6_rt_gc_min_interval = HZ / 2;
net->ipv6.sysctl.ip6_rt_gc_timeout = 60*HZ;
net->ipv6.sysctl.ip6_rt_gc_interval = 30*HZ;

The above patch resolved it for the Ipv6 reproducer.

Would that be sufficient?

> I do not believe the suggested flag is the right change.

Regards

Jon