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Message-Id: <1459463081-20206-1-git-send-email-dsa@cumulusnetworks.com>
Date: Thu, 31 Mar 2016 15:24:41 -0700
From: David Ahern <dsa@...ulusnetworks.com>
To: netdev@...r.kernel.org
Cc: ja@....bg, David Ahern <dsa@...ulusnetworks.com>
Subject: [PATCH v2 net-next] net: ipv4: Consider unreachable nexthops in multipath routes
Multipath route lookups should consider knowledge about next hops and not
select a hop that is known to be failed.
Example:
[h2] [h3] 15.0.0.5
| |
3| 3|
[SP1] [SP2]--+
1 2 1 2
| | /-------------+ |
| \ / |
| X |
| / \ |
| / \---------------\ |
1 2 1 2
12.0.0.2 [TOR1] 3-----------------3 [TOR2] 12.0.0.3
4 4
\ /
\ /
\ /
-------| |-----/
1 2
[TOR3]
3|
|
[h1] 12.0.0.1
host h1 with IP 12.0.0.1 has 2 paths to host h3 at 15.0.0.5:
root@h1:~# ip ro ls
...
12.0.0.0/24 dev swp1 proto kernel scope link src 12.0.0.1
15.0.0.0/16
nexthop via 12.0.0.2 dev swp1 weight 1
nexthop via 12.0.0.3 dev swp1 weight 1
...
If the link between tor3 and tor1 is down and the link between tor1
and tor2 then tor1 is effectively cut-off from h1. Yet the route lookups
in h1 are alternating between the 2 routes: ping 15.0.0.5 gets one and
ssh 15.0.0.5 gets the other. Connections that attempt to use the
12.0.0.2 nexthop fail since that neighbor is not reachable:
root@h1:~# ip neigh show
...
12.0.0.3 dev swp1 lladdr 00:02:00:00:00:1b REACHABLE
12.0.0.2 dev swp1 FAILED
...
The failed path can be avoided by considering known neighbor information
when selecting next hops. If the neighbor lookup fails we have no
knowledge about the nexthop, so give it a shot. If there is an entry
then only select the nexthop if the state is sane. This is similar to
what fib_detect_death does.
To maintain backward compatibility use of the neighbor information is
based on a new sysctl, fib_multipath_use_neigh.
Signed-off-by: David Ahern <dsa@...ulusnetworks.com>
---
v2
- use rcu locking to avoid refcnts per Eric's suggestion
- only consider neighbor info for nh_scope == RT_SCOPE_LINK per Julian's
comment
- drop the 'state == NUD_REACHABLE' from the state check since it is
part of NUD_VALID (comment from Julian)
- wrapped the use of the neigh in a sysctl
Documentation/networking/ip-sysctl.txt | 10 ++++++++++
include/net/netns/ipv4.h | 3 +++
net/ipv4/fib_semantics.c | 32 ++++++++++++++++++++++++++++++--
net/ipv4/sysctl_net_ipv4.c | 11 +++++++++++
4 files changed, 54 insertions(+), 2 deletions(-)
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index b183e2b606c8..5b316d33a23f 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -63,6 +63,16 @@ fwmark_reflect - BOOLEAN
fwmark of the packet they are replying to.
Default: 0
+fib_multipath_use_neigh - BOOLEAN
+ Use status of existing neighbor entry when determining nexthop for
+ multipath routes. If disabled neighbor information is not used then
+ packets could be directed to a dead nexthop. Only valid for kernels
+ built with CONFIG_IP_ROUTE_MULTIPATH enabled.
+ Default: 0 (disabled)
+ Possible values:
+ 0 - disabled
+ 1 - enabled
+
route/max_size - INTEGER
Maximum number of routes allowed in the kernel. Increase
this when using large numbers of interfaces and/or routes.
diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index a69cde3ce460..d061ffeb1e71 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -133,6 +133,9 @@ struct netns_ipv4 {
struct fib_rules_ops *mr_rules_ops;
#endif
#endif
+#ifdef CONFIG_IP_ROUTE_MULTIPATH
+ int sysctl_fib_multipath_use_neigh;
+#endif
atomic_t rt_genid;
};
#endif
diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index d97268e8ff10..6d423faff0ce 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -1559,17 +1559,45 @@ int fib_sync_up(struct net_device *dev, unsigned int nh_flags)
}
#ifdef CONFIG_IP_ROUTE_MULTIPATH
+static bool fib_good_nh(const struct fib_nh *nh, struct net_device *dev)
+{
+ struct neighbour *n = NULL;
+ int state = NUD_NONE;
+
+ if (nh->nh_scope == RT_SCOPE_LINK) {
+ rcu_read_lock_bh();
+
+ n = __neigh_lookup_noref(&arp_tbl, &nh->nh_gw, dev);
+ if (n)
+ state = n->nud_state;
+
+ rcu_read_unlock_bh();
+ }
+
+ /* outside of rcu locking using n only as a boolean
+ * on whether a neighbor entry existed
+ */
+ if (!n || (state & NUD_VALID))
+ return true;
+
+ return false;
+}
void fib_select_multipath(struct fib_result *res, int hash)
{
struct fib_info *fi = res->fi;
+ struct net_device *dev = fi->fib_dev;
+ struct net *net = fi->fib_net;
for_nexthops(fi) {
if (hash > atomic_read(&nh->nh_upper_bound))
continue;
- res->nh_sel = nhsel;
- return;
+ if (!net->ipv4.sysctl_fib_multipath_use_neigh ||
+ fib_good_nh(nh, dev)) {
+ res->nh_sel = nhsel;
+ return;
+ }
} endfor_nexthops(fi);
/* Race condition: route has just become dead. */
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 1e1fe6086dd9..bb0419582b8d 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -960,6 +960,17 @@ static struct ctl_table ipv4_net_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec,
},
+#ifdef CONFIG_IP_ROUTE_MULTIPATH
+ {
+ .procname = "fib_multipath_use_neigh",
+ .data = &init_net.ipv4.sysctl_fib_multipath_use_neigh,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &one,
+ },
+#endif
{ }
};
--
1.9.1
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