[PATCH net 1/2] net/sched: fq_codel: match the no-drop threshold to the packet size

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

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) <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_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 969b2510b0b8..e6c87a32950f 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c
@@ -517,8 +517,8 @@ 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);
-- 
2.43.0