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

[Ido Schimmel <idosch@idosch.org>]

On Tue, Oct 29, 2024 at 05:22:23PM -0600, David Ahern wrote:
> 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.

Doesn't IPv6 suffer from a similar problem?

> 
> 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@verdict.gg>

Thanks for the detailed commit message