[PATCH net 2/2] net/sched: sch_codel: match the no-drop threshold to the packet size

[Jamal Hadi Salim <jhs@mojatatu.com>]

commit 6439461f1618 ("net/sched: sch_codel: clamp default mtu to avoid
disabling CoDel") clamped q->params.mtu to [256, 1 << 20]. The value is
the CoDel no-drop threshold (codel_impl.h "*backlog <= params->mtu"), so
on a link whose maximum transmitted packet size is below 256 the floor
extends CoDel's minimum-backlog exemption beyond one packet and delays
drop or mark eligibility by several small packets.

Keep the upper bound that guards the original overflow (psched_mtu()
wrapping to ~2 GiB on a huge-MTU device) but drop the 256 floor, so the
threshold tracks the real device packet size.

Conditions to recreate the bug: attach a codel qdisc on a link whose
MTU plus hard_header_len is below 256 (e.g. a CAN interface). At that
MTU the no-drop threshold must equal the device MTU plus its
hard-header length; before this patch it was forced to 256.
Requires CAP_NET_ADMIN in a user namespace.

Fixes: 6439461f1618 ("net/sched: sch_codel: clamp default mtu to avoid disabling CoDel")
Reported-by: Sashiko (nipa) <sashiko-bot@kernel.org>
Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260819143136.57350-1-jhs@mojatatu.com
Tested-by: hybris <hybris@mojatatu.ai>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/sch_codel.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_codel.c b/net/sched/sch_codel.c
index 6aa5829d6961..a1ba2e355f2d 100644
--- a/net/sched/sch_codel.c
+++ b/net/sched/sch_codel.c
@@ -205,7 +205,7 @@ static int codel_init(struct Qdisc *sch, struct nlattr *opt,
 	codel_params_init(&q->params);
 	codel_vars_init(&q->vars);
 	codel_stats_init(&q->stats);
-	q->params.mtu = clamp_t(u32, psched_mtu(qdisc_dev(sch)), 256, 1 << 20);
+	q->params.mtu = min_t(u32, psched_mtu(qdisc_dev(sch)), 1 << 20);
 
 	if (opt) {
 		int err = codel_change(sch, opt, extack);
-- 
2.43.0