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On Monday 02/02 at 16:52 -0800, Alex Gartrell wrote:
> Hello Shengyong,
>
> > diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> > index b2614b2..b80317a 100644
> > --- a/net/ipv6/route.c
> > +++ b/net/ipv6/route.c
> > @@ -1136,6 +1136,9 @@ static void ip6_rt_update_pmtu(struct
> dst_entry *dst, struct sock *sk,
> > {
> > struct rt6_info *rt6 = (struct rt6_info*)dst;
> >
> > + if (rt6->rt6i_flags & RTF_LOCAL)
> > + return;
> > +
> > dst_confirm(dst);
> > if (mtu < dst_mtu(dst) && rt6->rt6i_dst.plen == 128) {
> > struct net *net = dev_net(dst->dev);
> >
> > So is this modification correct? Or how can we avoid such expiring?
>
> FWIW, we encountered this problem with IPVS tunneling. Here's a
> patch done by Calvin (cc'ed) that fixes my attempted fix for this.
> We're not particularly proud of this...
>
> At a high level, I don't think the RTF_LOCAL check was sufficient,
> but I didn't investigate deeply enough and hopefully Calvin can say
> why.
I honestly didn't spend much time at all finding the underlying cause
because it appeared to be fixed upstream: on 3.19-rc5 you get all 3
expected routes after the last step of my repro below. I just really
needed to get this working at the time, and the gross disgusting
horrible ugly awful [more negative adjectives] patch included below made
it work.
FWIW, the explanation I wrote down in my notes was:
"The absence of RTF_NONEXTHOP is causing COWs to happen, which are
always marked as RTF_CACHE. Somehow that's screwing things up in
rt6_do_redirect()"
That could be BS though, I don't at all remember how I came to that
conclusion.
(/me resolves to write better notes in the future...)
Here's how to get the weird behavior on 3.10 (+stable):
$ sudo ip addr add local 4444::1 dev lo
### Now I have 2 routes in /proc/net/ipv6_route, a local and a non-local
### Both have the RTF_NONEXTHOP flag set (0x00200000)
$ sudo ip route add local 4444::1 dev lo
### Now I have 3 routes in /proc/net/ipv6_route to 4444::1
### Notice the new route does NOT have the RTF_NONEXTHOP flag set
$ sudo ip addr del local 4444::1 dev lo
### Now I just have the one route I created before
$ sudo ip addr add local 4444::1 dev lo
### And now I have 3 routes again
$ sudo ping6 4444::1
[blah blah blah successful ping]
$ sudo ip addr del local 4444::1 dev lo
$ sudo ip addr add local 4444::1 dev lo
### Still have 3 routes
$ sudo ip addr del local 4444::1 dev lo
### Now I just have my one route yet again
### Now, *without the address on lo*, talk to it (it works), then re-add it
$ ping6 4444::1
[blah blah blah successful ping]
$ sudo ip addr add local 4444::1 dev lo
### Now I only have 2 routes... WAT!?
### Notice the LOCAL (0x80000000) route doesn't have the RTF_NONEXTHOP flag set
Thanks,
Calvin
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index f14d49b..c607a42 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -1159,18 +1159,18 @@ static void ip6_rt_update_pmtu(struct
> dst_entry *dst, struct sock *sk,
> }
> dst_metric_set(dst, RTAX_MTU, mtu);
>
> - /* FACEBOOK HACK: We need to not expire local non-expiring
> - * routes so that we don't accidentally start blackholing
> - * ipvs traffic when we happen to use it locally for
> - * healthchecking (see ip_vs_xmit.c --
> - * __ip_vs_get_out_rt_v6 invokes update_pmtu if the rt is
> - * associated with a socket)
> - * Alex Gartrell <agartrell@xxxxxx>
> + /*
> + * FACEBOOK HACK: Only expire routes that aren't destined for
> + * the loopback interface.
> + *
> + * This prevents the strange route coalescing that happens
> when
> + * you add an address to the loopback that had a route that
> had
> + * been used when the address didn't exist from getting
> expired
> + * and causing packet loss in shiv.
> */
> - if (!(rt6->rt6i_flags & RTF_LOCAL) ||
> - (rt6->rt6i_flags & (RTF_EXPIRES | RTF_CACHE)))
> - rt6_update_expires(
> - rt6, net->ipv6.sysctl.ip6_rt_mtu_expires);
> + if (!(dst->dev->flags & IFF_LOOPBACK))
> + rt6_update_expires(rt6,
> + net->ipv6.sysctl.ip6_rt_mtu_expires);
> }
> }
>
>
> Cheers,
> --
> Alex Gartrell <agartrell@xxxxxx>
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