netdev - Re: IP fragmentation broken in 3.6-rc ?

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Date:	Tue, 21 Aug 2012 19:23:44 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Julian Anastasov <ja@....bg>
Cc:	Sylvain Munaut <s.munaut@...tever-company.com>,
	netdev@...r.kernel.org
Subject: Re: IP fragmentation broken in 3.6-rc ?

On Tue, 2012-08-21 at 20:18 +0300, Julian Anastasov wrote:
> 	Hello,
> 
> On Tue, 21 Aug 2012, Eric Dumazet wrote:

> > My first patch was :
> > 
> > diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> > index e4ba974..9858714 100644
> > --- a/net/ipv4/route.c
> > +++ b/net/ipv4/route.c
> > @@ -956,6 +956,7 @@ static void ip_rt_update_pmtu(struct dst_entry *dst, struct sock *sk,
> >  		dst->obsolete = DST_OBSOLETE_KILL;
> >  	} else {
> >  		rt->rt_pmtu = mtu;
> > +		rt->dst.expires = 0;
> >  		dst_set_expires(&rt->dst, ip_rt_mtu_expires);
> 
> 	This is better, does not break ipv4_link_failure.
> There is a little race some ipv4_mtu() user to see
> rt_pmtu != 0 and dst.expires = 0 and to fail in time_after_eq
> test. May be that is why dst.expires is never set to 0.
> But I still don't understand what both changes fix.

In fact we re-enter ip_rt_update_pmtu() if we receive a second ICMP,
and rt->rt_pmtu is already set, but dst is expired.

Thats why Sylvain said it was not happening in the 10 minutes following
boot.

So calling again dst_set_expires(&rt->dst, ip_rt_mtu_expires) does
nothing : rt_pmtu is ignored because dst.expires is too old.

Maybe we should just do :

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index e4ba974..d0181e2 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -952,7 +952,7 @@ static void ip_rt_update_pmtu(struct dst_entry *dst, struct sock *sk,
 	ip_rt_build_flow_key(&fl4, sk, skb);
 	mtu = __ip_rt_update_pmtu(rt, &fl4, mtu);
 
-	if (!rt->rt_pmtu) {
+	if (!rt->rt_pmtu || time_after_eq(jiffies, rt->dst.expires)) {
 		dst->obsolete = DST_OBSOLETE_KILL;
 	} else {
 		rt->rt_pmtu = mtu;


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