Re: [PATCH RFC net-next] net: ipvs: Adjust gso_size for IPPROTO_TCP

To: Martin KaFai Lau <kafai@xxxxxx>
Subject: Re: [PATCH RFC net-next] net: ipvs: Adjust gso_size for IPPROTO_TCP
Cc: netdev@xxxxxxxxxxxxxxx, David Ahern <dsahern@xxxxxxxxx>, Tom Herbert <tom@xxxxxxxxxxxxxxx>, Eric Dumazet <eric.dumazet@xxxxxxxxx>, Nikita Shirokov <tehnerd@xxxxxx>, kernel-team@xxxxxx, lvs-devel@xxxxxxxxxxxxxxx
From: Julian Anastasov <ja@xxxxxx>
Date: Sat, 5 May 2018 15:58:25 +0300 (EEST)

On Thu, 3 May 2018, Martin KaFai Lau wrote:

> > - when exactly we start to use the new PMTU, eg. what happens
> > in case socket caches the route, whether route is killed via
> > dst->obsolete. Or may be while the PMTU expiration is handled
> > per-packet, the PMTU change is noticed only on ICMP...
> Before sk can reuse its dst cache, the sk will notice
> its dst cache is no longer valid by calling dst_check().
> dst_check() should return NULL which is one of the side
> effect of the earlier update_pmtu().  This dst_check()
> is usually only called when the sk needs to do output,
> so the new PMTU route (i.e. the RTF_CACHE IPv6 route)
> only have effect to the later packets.

        I checked again the code and it looks like sockets
are forced to use new exceptional route (RTF_CACHE/fnhe) via
dst_check only when the PMTU update should move them away
from old non-exceptional routes. Later, if PMTU is
reduced/updated this is noticed for every packet via dst_mtu,
as in the case with TCP.

        So, except the RTF_LOCAL check in __ip6_rt_update_pmtu
we should have no other issues. Only one minor bit is strange to me,
why rt6_insert_exception warns for RTF_PCPU if rt6_cache_allowed_for_pmtu
allows it when returning true... 

        Also, commit 0d3f6d297bfb allows rt6_do_update_pmtu() for
routes without RTF_CACHE, RTF_PCPU and rt6i_node. Should we
restrict rt6_do_update_pmtu only to RTF_CACHE routes?

        if (!rt6_cache_allowed_for_pmtu(rt6)) {
-               rt6_do_update_pmtu(rt6, mtu);
-               /* update rt6_ex->stamp for cache */
-               if (rt6->rt6i_flags & RTF_CACHE)
+               if (rt6->rt6i_flags & RTF_CACHE) {
+                       rt6_do_update_pmtu(rt6, mtu);
+                       /* update rt6_ex->stamp for cache */
+               }
        } else if (daddr) {


Julian Anastasov <ja@xxxxxx>
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