net/sched: sch_codel: match the no-drop threshold to the packet size

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commit 6439461f16 ("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: 6439461f16 ("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
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/QDISC-34MS.v1.20260925165535@mojatatu.com.2
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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This commit is contained in:

Jamal Hadi Salim 2026-09-26 14:03:28 -04:00 • committed by Paolo Abeni

parent d031465acc

commit 54518e0e82

1 changed files with 1 additions and 1 deletions

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net/sched/sch_codel.c

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