netdev - [PATCH net 1/2] net: fib: restore ECMP balance from loopback

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Message-ID: <20251213135849.2054677-1-vadim.fedorenko@linux.dev>
Date: Sat, 13 Dec 2025 13:58:48 +0000
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: "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.ord>
Cc: Shuah Khan <shuah@...nel.org>,
	Ido Schimmel <idosch@...dia.com>,
	netdev@...r.kernel.org,
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Subject: [PATCH net 1/2] net: fib: restore ECMP balance from loopback

Preference of nexthop with source address broke ECMP for packets with
source address from loopback interface. 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 lo:

before:
   done | grep veth | awk ' {print $(NF-2)}' | sort | uniq -c:
    255 veth3

after:
   done | grep veth | awk ' {print $(NF-2)}' | sort | uniq -c:
    122 veth1
    133 veth3

Fixes: 32607a332cfe ("ipv4: prefer multipath nexthop that matches source address")
Signed-off-by: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
---
 net/ipv4/fib_semantics.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index a5f3c8459758..c54b4ad9c280 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -2165,9 +2165,9 @@ static bool fib_good_nh(const struct fib_nh *nh)
 void fib_select_multipath(struct fib_result *res, int hash,
 			  const struct flowi4 *fl4)
 {
+	bool first = false, found = false;
 	struct fib_info *fi = res->fi;
 	struct net *net = fi->fib_net;
-	bool found = false;
 	bool use_neigh;
 	__be32 saddr;
 
@@ -2190,23 +2190,24 @@ 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) {
 			res->nh_sel = nhsel;
 			res->nhc = &nexthop_nh->nh_common;
-			found = !saddr || nexthop_nh->nh_saddr == saddr;
+			return;
 		}
 
-		if (hash > nh_upper_bound)
-			continue;
-
-		if (!saddr || nexthop_nh->nh_saddr == saddr) {
+		if (!first) {
 			res->nh_sel = nhsel;
 			res->nhc = &nexthop_nh->nh_common;
-			return;
+			first = true;
 		}
 
-		if (found)
-			return;
+		if (found || hash > nh_upper_bound)
+			continue;
+
+		res->nh_sel = nhsel;
+		res->nhc = &nexthop_nh->nh_common;
+		found = true;
 
 	} endfor_nexthops(fi);
 }
-- 
2.47.3