netdev - Re: [PATCH net-next v8] net: ipv4: Cache pmtu for all packet paths if multipath enabled

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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>