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) Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260819143136.57350-1-jhs@mojatatu.com Tested-by: hybris Signed-off-by: Jamal Hadi Salim --- 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