6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jamal Hadi Salim [ Upstream commit d031465acc362866f27e4f8f79cfecf1037fe2e5 ] commit d9ebd8f9aa8b ("net/sched: fq_codel: clamp default quantum and mtu") clamped both q->quantum and q->cparams.mtu to [256, FQ_CODEL_QUANTUM_MAX]. The two fields mean different things: quantum is a DRR credit that wants the 256 floor, but cparams.mtu is the CoDel no-drop threshold (codel_impl.h "*backlog <= params->mtu"). 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. Split the clamp. quantum keeps [256, FQ_CODEL_QUANTUM_MAX]; cparams.mtu tracks psched_mtu() (the device MTU plus its hard-header length) with only the upper bound that guards the original overflow (psched_mtu() wrapping to ~2 GiB on a huge-MTU device). Conditions to recreate the bug: attach an fq_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. Basic Testing done: with dev->mtu=100 and hard_header_len=14, a return probe on fq_codel_init() observed cparams.mtu change from 256 to 114 Fixes: d9ebd8f9aa8b ("net/sched: fq_codel: clamp default quantum and mtu") Reported-by: Sashiko (nipa) Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260819143136.57350-1-jhs@mojatatu.com Signed-off-by: Jamal Hadi Salim Link: https://patch.msgid.link/QDISC-34MS.v1.20260925165535@mojatatu.com Signed-off-by: Paolo Abeni Signed-off-by: Sasha Levin --- net/sched/sch_fq_codel.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c index ac2f643dd8427..08cb94bf5950b 100644 --- a/net/sched/sch_fq_codel.c +++ b/net/sched/sch_fq_codel.c @@ -503,8 +503,8 @@ static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt, q->flows_cnt = 1024; q->memory_limit = 32 << 20; /* 32 MBytes */ q->drop_batch_size = 64; - mtu = clamp_t(u32, psched_mtu(qdisc_dev(sch)), 256, FQ_CODEL_QUANTUM_MAX); - q->quantum = mtu; + mtu = min_t(u32, psched_mtu(qdisc_dev(sch)), FQ_CODEL_QUANTUM_MAX); + q->quantum = clamp_t(u32, mtu, 256, FQ_CODEL_QUANTUM_MAX); INIT_LIST_HEAD(&q->new_flows); INIT_LIST_HEAD(&q->old_flows); codel_params_init(&q->cparams); -- 2.53.0