netdev - [PATCH v2] net: ipv4: Cache pmtu for all packet paths if multipath enabled

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Message-ID: <20241029185350.304763-1-deliran@verdict.gg>
Date: Tue, 29 Oct 2024 18:46:45 +0000
From: Vladimir Vdovin <deliran@...dict.gg>
To: netdev@...r.kernel.org,
	dsahern@...nel.org,
	davem@...emloft.net
Cc: Vladimir Vdovin <deliran@...dict.gg>
Subject: [PATCH v2] net: ipv4: Cache pmtu for all packet paths if multipath enabled

Check number of paths by fib_info_num_path(),
and update_or_create_fnhe() for every path.
Problem is that pmtu is cached only for the oif
that has received icmp message "need to frag",
other oifs will still try to use "default" iface mtu.

V2:
  - fix fib_info_num_path parameter pass

An example topology showing the problem:

                    |  host1
                +---------+
                |  dummy0 | 10.179.20.18/32  mtu9000
                +---------+
        +-----------+----------------+
    +---------+                     +---------+
    | ens17f0 |  10.179.2.141/31    | ens17f1 |  10.179.2.13/31
    +---------+                     +---------+
        |    (all here have mtu 9000)    |
    +------+                         +------+
    | ro1  |  10.179.2.140/31        | ro2  |  10.179.2.12/31
    +------+                         +------+
        |                                |
---------+------------+-------------------+------
                        |
                    +-----+
                    | ro3 | 10.10.10.10  mtu1500
                    +-----+
                        |
    ========================================
                some networks
    ========================================
                        |
                    +-----+
                    | eth0| 10.10.30.30  mtu9000
                    +-----+
                        |  host2

host1 have enabled multipath and
sysctl net.ipv4.fib_multipath_hash_policy = 1:

default proto static src 10.179.20.18
        nexthop via 10.179.2.12 dev ens17f1 weight 1
        nexthop via 10.179.2.140 dev ens17f0 weight 1

When host1 tries to do pmtud from 10.179.20.18/32 to host2,
host1 receives at ens17f1 iface an icmp packet from ro3 that ro3 mtu=1500.
And host1 caches it in nexthop exceptions cache.

Problem is that it is cached only for the iface that has received icmp,
and there is no way that ro3 will send icmp msg to host1 via another path.

Host1 now have this routes to host2:

ip r g 10.10.30.30 sport 30000 dport 443
10.10.30.30 via 10.179.2.12 dev ens17f1 src 10.179.20.18 uid 0
    cache expires 521sec mtu 1500

ip r g 10.10.30.30 sport 30033 dport 443
10.10.30.30 via 10.179.2.140 dev ens17f0 src 10.179.20.18 uid 0
    cache

So when host1 tries again to reach host2 with mtu>1500,
if packet flow is lucky enough to be hashed with oif=ens17f1 its ok,
if oif=ens17f0 it blackholes and still gets icmp msgs from ro3 to ens17f1,
until lucky day when ro3 will send it through another flow to ens17f0.

Signed-off-by: Vladimir Vdovin <deliran@...dict.gg>
---
 net/ipv4/route.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 723ac9181558..cbb2e22510ce 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1027,10 +1027,23 @@ static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu)
 		struct fib_nh_common *nhc;
 
 		fib_select_path(net, &res, fl4, NULL);
+#ifdef CONFIG_IP_ROUTE_MULTIPATH
+		if (fib_info_num_path(res.fi) > 1) {
+			int nhsel;
+
+			for (nhsel = 0; nhsel < fib_info_num_path(res.fi); nhsel++) {
+				nhc = fib_info_nhc(res.fi, nhsel);
+				update_or_create_fnhe(nhc, fl4->daddr, 0, mtu, lock,
+					jiffies + net->ipv4.ip_rt_mtu_expires);
+			}
+			goto rcu_unlock;
+		}
+#endif /* CONFIG_IP_ROUTE_MULTIPATH */
 		nhc = FIB_RES_NHC(res);
 		update_or_create_fnhe(nhc, fl4->daddr, 0, mtu, lock,
 				      jiffies + net->ipv4.ip_rt_mtu_expires);
 	}
+rcu_unlock:
 	rcu_read_unlock();
 }
 

base-commit: 66600fac7a984dea4ae095411f644770b2561ede
-- 
2.43.0