From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A5A583ADBA4; Sat, 17 Oct 2026 16:59:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792256371; cv=none; b=LHbqgh9YVL83IU8+Ku7vueO6YmCGBJZ92XSdgxjbe26GgyFvN5Brr9ooN5CV6Ha2o045GFQxY5CmT9UYqBA66qm0xILZ7EXK1v193/1YiiETLY+sv8q43lWd441+ME0UiQLJXXU/Dhu8CBSmaWYYliXa3mYHeRWUcsOf1xHO3L8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792256371; c=relaxed/simple; bh=bKgVKB2N2gU7sX6tQRIuIuqxV4l65NHUldPhH0n4RKg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=KcgFmAxoEx/QmE2F+TS9T6QYPKfFIfAo0gVjW/uxHZ6hhRBWsL1kOatlOhDm9ynkIN9UdNaHAmZmdtmu80qrqWG07elNTe468KAn1pkxkkCFyzREpjVJX4xWo4UZrR7TXnxn8+B8GE0gFmZ8oaGdBFHLOguNCyH4E0m2P9u6jt4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=WrPM6E/z; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="WrPM6E/z" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F15621F000C4; Sat, 17 Oct 2026 16:59:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1792256370; bh=S59ywalC8dDGvURTb+tMeWoeIfThLE/95aq45BDGPGk=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=WrPM6E/zqEdZTEq9VgbTKWHjGwzbxE5b5HtmJQIISpPipyF86WdUu0S22BBGAhthQ fb1JFl4lmt8fL0H7zByhSGWzg1PEojzh+/43jvJ/mmrsvEpiGASZL/7Frl6B2rSM3T L1x59OEg4B1n4PYSJ8NAUFvgYWSnHO4+eNKnqAig= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, "Sashiko (nipa)" , Jamal Hadi Salim , Paolo Abeni , Sasha Levin Subject: [PATCH 6.6 0215/1674] net/sched: fq_codel: match the no-drop threshold to the packet size Date: Sat, 17 Oct 2026 17:41:27 +0200 Message-ID: <20261017160436.668802226@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261017160431.119607952@linuxfoundation.org> References: <20261017160431.119607952@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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