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Message-ID: <1c6ba073-79e5-461b-ae76-4ef22fe04632@linux.dev>
Date: Sun, 21 Dec 2025 18:49:53 +0000
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: Ido Schimmel <idosch@...dia.com>,
Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern
<dsahern@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
Willem de Bruijn <willemb@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
Shuah Khan <shuah@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH net v2 1/2] net: fib: restore ECMP balance from loopback
On 21/12/2025 16:58, Ido Schimmel wrote:
> On Sun, Dec 21, 2025 at 10:55:15AM -0500, Willem de Bruijn wrote:
>> Vadim Fedorenko wrote:
>>> Preference of nexthop with source address broke ECMP for packets with
>>> source addresses which are not in the broadcast domain, but rather added
>>> to loopback/dummy interfaces. Original behaviour was to balance over
>>> nexthops while now it uses the latest nexthop from the group.
>>>
>>> For the case with 198.51.100.1/32 assigned to dummy0 and routed using
>>> 192.0.2.0/24 and 203.0.113.0/24 networks:
>>>
>>> 2: dummy0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 1000
>>> link/ether d6:54:8a:ff:78:f5 brd ff:ff:ff:ff:ff:ff
>>> inet 198.51.100.1/32 scope global dummy0
>>> valid_lft forever preferred_lft forever
>>> 7: veth1@if6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
>>> link/ether 06:ed:98:87:6d:8a brd ff:ff:ff:ff:ff:ff link-netnsid 0
>>> inet 192.0.2.2/24 scope global veth1
>>> valid_lft forever preferred_lft forever
>>> inet6 fe80::4ed:98ff:fe87:6d8a/64 scope link proto kernel_ll
>>> valid_lft forever preferred_lft forever
>>> 9: veth3@if8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
>>> link/ether ae:75:23:38:a0:d2 brd ff:ff:ff:ff:ff:ff link-netnsid 0
>>> inet 203.0.113.2/24 scope global veth3
>>> valid_lft forever preferred_lft forever
>>> inet6 fe80::ac75:23ff:fe38:a0d2/64 scope link proto kernel_ll
>>> valid_lft forever preferred_lft forever
>>>
>>> ~ ip ro list:
>>> default
>>> nexthop via 192.0.2.1 dev veth1 weight 1
>>> nexthop via 203.0.113.1 dev veth3 weight 1
>>> 192.0.2.0/24 dev veth1 proto kernel scope link src 192.0.2.2
>>> 203.0.113.0/24 dev veth3 proto kernel scope link src 203.0.113.2
>>>
>>> before:
>>> for i in {1..255} ; do ip ro get 10.0.0.$i; done | grep veth | awk ' {print $(NF-2)}' | sort | uniq -c:
>>> 255 veth3
>>>
>>> after:
>>> for i in {1..255} ; do ip ro get 10.0.0.$i; done | grep veth | awk ' {print $(NF-2)}' | sort | uniq -c:
>>> 122 veth1
>>> 133 veth3
>
> The commit message only explains the problem, but not the solution...
Well, the solution is to try to restore original logic. But ok, I'll
explain it explicitly
>
>>>
>>> Fixes: 32607a332cfe ("ipv4: prefer multipath nexthop that matches source address")
>>> Signed-off-by: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
>>> ---
>>> v1 -> v2:
>>>
>>> - add score calculation for nexthop to keep original logic
>>> - adjust commit message to explain the config
>>> - use dummy device instead of loopback
>>> ---
>>>
>>> net/ipv4/fib_semantics.c | 24 ++++++++----------------
>>> 1 file changed, 8 insertions(+), 16 deletions(-)
>>>
>>> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
>>> index a5f3c8459758..4d3650d20ff2 100644
>>> --- a/net/ipv4/fib_semantics.c
>>> +++ b/net/ipv4/fib_semantics.c
>>> @@ -2167,8 +2167,8 @@ void fib_select_multipath(struct fib_result *res, int hash,
>>> {
>>> struct fib_info *fi = res->fi;
>>> struct net *net = fi->fib_net;
>>> - bool found = false;
>>> bool use_neigh;
>>> + int score = -1;
>>> __be32 saddr;
>>>
>>> if (unlikely(res->fi->nh)) {
>>> @@ -2180,7 +2180,7 @@ void fib_select_multipath(struct fib_result *res, int hash,
>>> saddr = fl4 ? fl4->saddr : 0;
>>>
>>> change_nexthops(fi) {
>>> - int nh_upper_bound;
>>> + int nh_upper_bound, nh_score = 0;
>>>
>>> /* Nexthops without a carrier are assigned an upper bound of
>>> * minus one when "ignore_routes_with_linkdown" is set.
>>> @@ -2190,24 +2190,16 @@ void fib_select_multipath(struct fib_result *res, int hash,
>>> (use_neigh && !fib_good_nh(nexthop_nh)))
>>> continue;
>>>
>>> - if (!found) {
>>> + if (saddr && nexthop_nh->nh_saddr == saddr)
>>> + nh_score += 2;
>>> + if (hash <= nh_upper_bound)
>>> + nh_score++;
>>> + if (score < nh_score) {
>>> res->nh_sel = nhsel;
>>> res->nhc = &nexthop_nh->nh_common;
>>> - found = !saddr || nexthop_nh->nh_saddr == saddr;
>>
>> if score == 3 return immediately?
>
> We can also return early in the input path (!saddr) when score is 1.
> This seems to work:
>
> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
> index 4d3650d20ff2..0caf38e44c73 100644
> --- a/net/ipv4/fib_semantics.c
> +++ b/net/ipv4/fib_semantics.c
> @@ -2197,6 +2197,8 @@ void fib_select_multipath(struct fib_result *res, int hash,
> if (score < nh_score) {
> res->nh_sel = nhsel;
> res->nhc = &nexthop_nh->nh_common;
> + if (nh_score == 3 || (!saddr && nh_score == 1))
> + return;
> score = nh_score;
> }
>
It makes sense to amortize the loop. Going to send v3
> Tested with net/fib_tests.sh and forwarding/router_multipath.sh
>
>>
>>> + score = nh_score;
>>> }
>>>
>>> - if (hash > nh_upper_bound)
>>> - continue;
>>> -
>>> - if (!saddr || nexthop_nh->nh_saddr == saddr) {
>>> - res->nh_sel = nhsel;
>>> - res->nhc = &nexthop_nh->nh_common;
>>> - return;
>>> - }
>>> -
>>> - if (found)
>>> - return;
>>> -
>>> } endfor_nexthops(fi);
>>> }
>>> #endif
>>> --
>>> 2.47.3
>>>
>>
>>
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