Re: [PATCH] selftests: net: ip_defrag: increase netdev_max_backlog

[Thadeu Lima de Souza Cascardo]

On Fri, Dec 06, 2019 at 11:50:15AM -0300, Thadeu Lima de Souza Cascardo wrote:
> On Fri, Dec 06, 2019 at 05:41:01AM -0800, Eric Dumazet wrote:
> > 
> > 
> > On 12/6/19 4:17 AM, Thadeu Lima de Souza Cascardo wrote:
> > > On Wed, Dec 04, 2019 at 12:03:57PM -0800, Eric Dumazet wrote:
> > >>
> > >>
> > >> On 12/4/19 11:53 AM, Thadeu Lima de Souza Cascardo wrote:
> > >>> When using fragments with size 8 and payload larger than 8000, the backlog
> > >>> might fill up and packets will be dropped, causing the test to fail. This
> > >>> happens often enough when conntrack is on during the IPv6 test.
> > >>>
> > >>> As the larger payload in the test is 10000, using a backlog of 1250 allow
> > >>> the test to run repeatedly without failure. At least a 1000 runs were
> > >>> possible with no failures, when usually less than 50 runs were good enough
> > >>> for showing a failure.
> > >>>
> > >>> As netdev_max_backlog is not a pernet setting, this sets the backlog to
> > >>> 1000 during exit to prevent disturbing following tests.
> > >>>
> > >>
> > >> Hmmm... I would prefer not changing a global setting like that.
> > >> This is going to be flaky since we often run tests in parallel (using different netns)
> > >>
> > >> What about adding a small delay after each sent packet ?
> > >>
> > >> diff --git a/tools/testing/selftests/net/ip_defrag.c b/tools/testing/selftests/net/ip_defrag.c
> > >> index c0c9ecb891e1d78585e0db95fd8783be31bc563a..24d0723d2e7e9b94c3e365ee2ee30e9445deafa8 100644
> > >> --- a/tools/testing/selftests/net/ip_defrag.c
> > >> +++ b/tools/testing/selftests/net/ip_defrag.c
> > >> @@ -198,6 +198,7 @@ static void send_fragment(int fd_raw, struct sockaddr *addr, socklen_t alen,
> > >>                 error(1, 0, "send_fragment: %d vs %d", res, frag_len);
> > >>  
> > >>         frag_counter++;
> > >> +       usleep(1000);
> > >>  }
> > >>  
> > >>  static void send_udp_frags(int fd_raw, struct sockaddr *addr,
> > >>
> > > 
> > > That won't work because the issue only shows when we using conntrack, as the
> > > packet will be reassembled on output, then fragmented again. When this happens,
> > > the fragmentation code is transmitting the fragments in a tight loop, which
> > > floods the backlog.
> > 
> > Interesting !
> > 
> > So it looks like the test is correct, and exposed a long standing problem in this code.
> > 
> > We should not adjust the test to some kernel-of-the-day-constraints, and instead fix the kernel bug ;)
> > 
> > Where is this tight loop exactly ?
> > 
> > If this is feeding/bursting ~1000 skbs via netif_rx() in a BH context, maybe we need to call a variant
> > that allows immediate processing instead of (ab)using the softnet backlog.
> > 
> > Thanks !
> 
> This is the loopback interface, so its xmit calls netif_rx. I suppose we would
> have the same problem with veth, for example.
> 
> So net/ipv6/ip6_output.c:ip6_fragment has this:
> 
> 		for (;;) {
> 			/* Prepare header of the next frame,
> 			 * before previous one went down. */
> 			if (iter.frag)
> 				ip6_fraglist_prepare(skb, &iter);
> 
> 			skb->tstamp = tstamp;
> 			err = output(net, sk, skb);
> 			if (!err)
> 				IP6_INC_STATS(net, ip6_dst_idev(&rt->dst),
> 					      IPSTATS_MIB_FRAGCREATES);
> 
> 			if (err || !iter.frag)
> 				break;
> 
> 			skb = ip6_fraglist_next(&iter);
> 		}
> 
> output is ip6_finish_output2, which will call neigh_output, which ends up
> calling dev_queue_xmit.
> 
> In this case, ip6_fragment is being called probably from rawv6_send_hdrinc ->
> dst_output -> ip6_output -> ip6_finish_output -> __ip6_finish_output ->
> ip6_fragment.
> 
> dst_output at rawv6_send_hdrinc is being called after netfilter
> NF_INET_LOCAL_OUT hook. That one is gathering the fragments and only accepting
> that last, reassembled skb, which causes ip6_fragment enter that loop.
> 
> So, basically, the easiest way to reproduce this is using this test with
> loopback and netfilter doing the reassembly during conntrack. I see some BH
> locks here and there, but I think this is just filling up the backlog too fast
> to give any chance for softirq to kick in.
> 
> I will see if I can reproduce this using routed veths.
> 

Confirmed that the same happens when using veth.

vethX (nsX) <-> veth1 (router) forwards through veth2 (router) <-> vethY (nsY)

With such a setup, when I send those fragments from nsX to nsY, they get
through, until I setup that same conntrack rule on the router. Then, increasing
netdev_max_backlog allows those fragments to go through again.

That at least seems to be a plausible scenario that we would like to fix, as
you said, instead of only making a test pass.

Next Monday, I may test anything you come up with.

Thanks.
Cascardo.