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Message-ID: <736cdd43-4c4b-4341-bd77-c9a365dec2e5@kernel.org>
Date: Tue, 29 Oct 2024 17:22:23 -0600
From: David Ahern <dsahern@...nel.org>
To: Vladimir Vdovin <deliran@...dict.gg>, netdev@...r.kernel.org,
davem@...emloft.net, Ido Schimmel <idosch@...sch.org>
Subject: Re: [PATCH] net: ipv4: Cache pmtu for all packet paths if multipath
enabled
On 10/29/24 9:21 AM, Vladimir Vdovin wrote:
> 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.
>
> 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
>
well known problem, and years ago I meant to send a similar patch.
Can you add a test case under selftests; you will see many pmtu,
redirect and multipath tests.
> 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..8eac6e361388 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(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:
compiler error when CONFIG_IP_ROUTE_MULTIPATH is not set.
> rcu_read_unlock();
> }
>
>
> base-commit: 66600fac7a984dea4ae095411f644770b2561ede
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