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

[David Ahern]

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 That "Route cache is full" message is now out of date since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?

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