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Message-ID: <843b0fd9-0c71-423e-a1a3-bc10e6f987ec@kernel.org>
Date: Wed, 6 Nov 2024 08:44:15 -0700
From: David Ahern <dsahern@...nel.org>
To: Vladimir Vdovin <deliran@...dict.gg>, netdev@...r.kernel.org,
davem@...emloft.net, idosch@...sch.org
Cc: edumazet@...gle.com, linux-kselftest@...r.kernel.org, kuba@...nel.org,
pabeni@...hat.com, shuah@...nel.org, horms@...nel.org, gnault@...hat.com
Subject: Re: [PATCH net-next v8] net: ipv4: Cache pmtu for all packet paths if
multipath enabled
On 11/6/24 6:30 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
>
> 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>
> ---
> V8:
> selftests in pmtu.sh:
> - Change var names from "dummy" to "host"
> - Fix errors caused by incorrect iface arguments pass
> - Add src addr to setup_multipath_new
> - Change multipath* func order
> - Change route_get_dst_exception() && route_get_dst_pmtu_from_exception()
> and arguments pass where they are used
> as Ido suggested in https://lore.kernel.org/all/ZykH_fdcMBdFgXix@shredder/
>
> V7:
> selftest in pmtu.sh:
> - add setup_multipath() with old and new nh tests
> - add global "dummy_v4" addr variables
> - add documentation
> - remove dummy netdev usage in mp nh test
> - remove useless sysctl opts in mp nh test
>
> V6:
> - make commit message cleaner
>
> V5:
> - make self test cleaner
>
> V4:
> - fix selftest, do route lookup before checking cached exceptions
>
> V3:
> - add selftest
> - fix compile error
>
> V2:
> - fix fib_info_num_path parameter pass
> ---
> net/ipv4/route.c | 13 +++
> tools/testing/selftests/net/pmtu.sh | 119 ++++++++++++++++++++++++----
> 2 files changed, 115 insertions(+), 17 deletions(-)
>
Reviewed-by: David Ahern <dsahern@...nel.org>
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