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 D27123909B5; Sat, 17 Oct 2026 17:28:01 +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=1792258083; cv=none; b=k2CNoyzAMqRYr8v6KMfqQiCutPToFa8ruaLLOQk+n6dya3QbFCWLlkn9268ZruADuebibfZ9Ofx5TXC1cF7IJWMRxHcy9Aj4S9gpRrzk8l6LX+x2IHKSWZunJJ7eRyirbt1Es92JgfY86VlPYdhZjYvW2ILb/b8iqTBzZODzrG8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792258083; c=relaxed/simple; bh=zU9wkeYT7zCwycc2MTVFjJieoUw87Hm2ikyBX0Qlu/0=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ssFJorw5nhDp86IdtSiYGG/u3oJ25ue2GtOMKWoHueN+9NhPYIUmwGhdssdOQRHGaRNCMjXrFDtC41VNl10ltdT/DCIqeMhicTflAYmcIJzoTCYUZPmUvT5SY5h6c1sQgCm5BjAYuhGS7PdHD1T7JMoy95yv1ZLaTl4bQV9M2ag= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=gWQsO3F7; 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="gWQsO3F7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 03B2D1F000C4; Sat, 17 Oct 2026 17:28:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1792258081; bh=RlvbUYKUHslh3S6aPCOjQaY0i0ifahji2jj7GdYDEuc=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=gWQsO3F75Lbukc1PKOZI/HTeyxeJELimAJvLe9OGz9eH/GrLwHoZHoWob1Y0l997d Zdz8wQGmDCYzeSBzCc3UEVL9Iwco4j/+31idYyM4kXYZUX1EOp6dbg1EdsA7dkFSKL Yp83898oqCClsNWrwS1coHo3ZmTP2oBIUWkAbPbc= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, William Liu , Savino Dicanosa , Jakub Kicinski , Sasha Levin Subject: [PATCH 6.6 0767/1674] net/sched: Fix backlog accounting in qdisc_dequeue_internal Date: Sat, 17 Oct 2026 17:50:39 +0200 Message-ID: <20261017160450.788992546@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: William Liu [ Upstream commit 52bf272636bda69587952b35ae97690b8dc89941 ] This issue applies for the following qdiscs: hhf, fq, fq_codel, and fq_pie, and occurs in their change handlers when adjusting to the new limit. The problem is the following in the values passed to the subsequent qdisc_tree_reduce_backlog call given a tbf parent: When the tbf parent runs out of tokens, skbs of these qdiscs will be placed in gso_skb. Their peek handlers are qdisc_peek_dequeued, which accounts for both qlen and backlog. However, in the case of qdisc_dequeue_internal, ONLY qlen is accounted for when pulling from gso_skb. This means that these qdiscs are missing a qdisc_qstats_backlog_dec when dropping packets to satisfy the new limit in their change handlers. One can observe this issue with the following (with tc patched to support a limit of 0): export TARGET=fq tc qdisc del dev lo root tc qdisc add dev lo root handle 1: tbf rate 8bit burst 100b latency 1ms tc qdisc replace dev lo handle 3: parent 1:1 $TARGET limit 1000 echo ''; echo 'add child'; tc -s -d qdisc show dev lo ping -I lo -f -c2 -s32 -W0.001 127.0.0.1 2>&1 >/dev/null echo ''; echo 'after ping'; tc -s -d qdisc show dev lo tc qdisc change dev lo handle 3: parent 1:1 $TARGET limit 0 echo ''; echo 'after limit drop'; tc -s -d qdisc show dev lo tc qdisc replace dev lo handle 2: parent 1:1 sfq echo ''; echo 'post graft'; tc -s -d qdisc show dev lo The second to last show command shows 0 packets but a positive number (74) of backlog bytes. The problem becomes clearer in the last show command, where qdisc_purge_queue triggers qdisc_tree_reduce_backlog with the positive backlog and causes an underflow in the tbf parent's backlog (4096 Mb instead of 0). To fix this issue, the codepath for all clients of qdisc_dequeue_internal has been simplified: codel, pie, hhf, fq, fq_pie, and fq_codel. qdisc_dequeue_internal handles the backlog adjustments for all cases that do not directly use the dequeue handler. The old fq_codel_change limit adjustment loop accumulated the arguments to the subsequent qdisc_tree_reduce_backlog call through the cstats field. However, this is confusing and error prone as fq_codel_dequeue could also potentially mutate this field (which qdisc_dequeue_internal calls in the non gso_skb case), so we have unified the code here with other qdiscs. Fixes: 2d3cbfd6d54a ("net_sched: Flush gso_skb list too during ->change()") Fixes: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM") Fixes: 10239edf86f1 ("net-qdisc-hhf: Heavy-Hitter Filter (HHF) qdisc") Signed-off-by: William Liu Reviewed-by: Savino Dicanosa Link: https://patch.msgid.link/20250812235725.45243-1-will@willsroot.io Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- include/net/sch_generic.h | 11 ++++++++--- net/sched/sch_codel.c | 12 +++++++----- net/sched/sch_fq.c | 12 +++++++----- net/sched/sch_fq_codel.c | 12 +++++++----- net/sched/sch_fq_pie.c | 12 +++++++----- net/sched/sch_hhf.c | 12 +++++++----- net/sched/sch_pie.c | 12 +++++++----- 7 files changed, 50 insertions(+), 33 deletions(-) diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h index 9d623fc8b1fc2..8b4b4809fa1cd 100644 --- a/include/net/sch_generic.h +++ b/include/net/sch_generic.h @@ -1077,12 +1077,17 @@ static inline struct sk_buff *qdisc_dequeue_internal(struct Qdisc *sch, bool dir skb = __skb_dequeue(&sch->gso_skb); if (skb) { sch->q.qlen--; + qdisc_qstats_backlog_dec(sch, skb); return skb; } - if (direct) - return __qdisc_dequeue_head(&sch->q); - else + if (direct) { + skb = __qdisc_dequeue_head(&sch->q); + if (skb) + qdisc_qstats_backlog_dec(sch, skb); + return skb; + } else { return sch->dequeue(sch); + } } static inline struct sk_buff *qdisc_dequeue_head(struct Qdisc *sch) diff --git a/net/sched/sch_codel.c b/net/sched/sch_codel.c index 1f30dc76d20c7..655eee9e629be 100644 --- a/net/sched/sch_codel.c +++ b/net/sched/sch_codel.c @@ -168,9 +168,9 @@ static const struct nla_policy codel_policy[TCA_CODEL_MAX + 1] = { static int codel_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { + unsigned int dropped_pkts = 0, dropped_bytes = 0; struct codel_sched_data *q = qdisc_priv(sch); struct nlattr *tb[TCA_CODEL_MAX + 1]; - unsigned int qlen, dropped = 0; int err; err = nla_parse_nested_deprecated(tb, TCA_CODEL_MAX, opt, @@ -204,15 +204,17 @@ static int codel_change(struct Qdisc *sch, struct nlattr *opt, if (tb[TCA_CODEL_ECN]) q->params.ecn = !!nla_get_u32(tb[TCA_CODEL_ECN]); - qlen = sch->q.qlen; while (sch->q.qlen > sch->limit) { struct sk_buff *skb = qdisc_dequeue_internal(sch, true); - dropped += qdisc_pkt_len(skb); - qdisc_qstats_backlog_dec(sch, skb); + if (!skb) + break; + + dropped_pkts++; + dropped_bytes += qdisc_pkt_len(skb); rtnl_qdisc_drop(skb, sch); } - qdisc_tree_reduce_backlog(sch, qlen - sch->q.qlen, dropped); + qdisc_tree_reduce_backlog(sch, dropped_pkts, dropped_bytes); sch_tree_unlock(sch); return 0; diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c index 91f5ef6be0f23..2223edfd7aa77 100644 --- a/net/sched/sch_fq.c +++ b/net/sched/sch_fq.c @@ -806,11 +806,11 @@ static const struct nla_policy fq_policy[TCA_FQ_MAX + 1] = { static int fq_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { + unsigned int dropped_pkts = 0, dropped_bytes = 0; struct fq_sched_data *q = qdisc_priv(sch); struct nlattr *tb[TCA_FQ_MAX + 1]; - int err, drop_count = 0; - unsigned drop_len = 0; u32 fq_log; + int err; err = nla_parse_nested_deprecated(tb, TCA_FQ_MAX, opt, fq_policy, NULL); @@ -900,16 +900,18 @@ static int fq_change(struct Qdisc *sch, struct nlattr *opt, err = fq_resize(sch, fq_log); sch_tree_lock(sch); } + while (sch->q.qlen > sch->limit) { struct sk_buff *skb = qdisc_dequeue_internal(sch, false); if (!skb) break; - drop_len += qdisc_pkt_len(skb); + + dropped_pkts++; + dropped_bytes += qdisc_pkt_len(skb); rtnl_kfree_skbs(skb, skb); - drop_count++; } - qdisc_tree_reduce_backlog(sch, drop_count, drop_len); + qdisc_tree_reduce_backlog(sch, dropped_pkts, dropped_bytes); sch_tree_unlock(sch); return err; diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c index 08cb94bf5950b..f618ec382081b 100644 --- a/net/sched/sch_fq_codel.c +++ b/net/sched/sch_fq_codel.c @@ -402,6 +402,7 @@ static const struct nla_policy fq_codel_policy[TCA_FQ_CODEL_MAX + 1] = { static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { + unsigned int dropped_pkts = 0, dropped_bytes = 0; struct fq_codel_sched_data *q = qdisc_priv(sch); struct nlattr *tb[TCA_FQ_CODEL_MAX + 1]; u32 quantum = 0; @@ -470,13 +471,14 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt, q->memory_usage > q->memory_limit) { struct sk_buff *skb = qdisc_dequeue_internal(sch, false); - q->cstats.drop_len += qdisc_pkt_len(skb); + if (!skb) + break; + + dropped_pkts++; + dropped_bytes += qdisc_pkt_len(skb); rtnl_kfree_skbs(skb, skb); - q->cstats.drop_count++; } - qdisc_tree_reduce_backlog(sch, q->cstats.drop_count, q->cstats.drop_len); - q->cstats.drop_count = 0; - q->cstats.drop_len = 0; + qdisc_tree_reduce_backlog(sch, dropped_pkts, dropped_bytes); sch_tree_unlock(sch); return 0; diff --git a/net/sched/sch_fq_pie.c b/net/sched/sch_fq_pie.c index 75d6e7716ae8e..d7c144af98916 100644 --- a/net/sched/sch_fq_pie.c +++ b/net/sched/sch_fq_pie.c @@ -284,10 +284,9 @@ static struct sk_buff *fq_pie_qdisc_dequeue(struct Qdisc *sch) static int fq_pie_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { + unsigned int dropped_pkts = 0, dropped_bytes = 0; struct fq_pie_sched_data *q = qdisc_priv(sch); struct nlattr *tb[TCA_FQ_PIE_MAX + 1]; - unsigned int len_dropped = 0; - unsigned int num_dropped = 0; int err; err = nla_parse_nested(tb, TCA_FQ_PIE_MAX, opt, fq_pie_policy, extack); @@ -366,11 +365,14 @@ static int fq_pie_change(struct Qdisc *sch, struct nlattr *opt, while (sch->q.qlen > sch->limit) { struct sk_buff *skb = qdisc_dequeue_internal(sch, false); - len_dropped += qdisc_pkt_len(skb); - num_dropped += 1; + if (!skb) + break; + + dropped_pkts++; + dropped_bytes += qdisc_pkt_len(skb); rtnl_kfree_skbs(skb, skb); } - qdisc_tree_reduce_backlog(sch, num_dropped, len_dropped); + qdisc_tree_reduce_backlog(sch, dropped_pkts, dropped_bytes); sch_tree_unlock(sch); return 0; diff --git a/net/sched/sch_hhf.c b/net/sched/sch_hhf.c index e1e8527e45192..17027d1f4d11a 100644 --- a/net/sched/sch_hhf.c +++ b/net/sched/sch_hhf.c @@ -536,9 +536,9 @@ static const struct nla_policy hhf_policy[TCA_HHF_MAX + 1] = { static int hhf_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { + unsigned int dropped_pkts = 0, dropped_bytes = 0; struct hhf_sched_data *q = qdisc_priv(sch); struct nlattr *tb[TCA_HHF_MAX + 1]; - unsigned int qlen, prev_backlog; int err; u64 non_hh_quantum; u32 new_quantum = q->quantum; @@ -585,15 +585,17 @@ static int hhf_change(struct Qdisc *sch, struct nlattr *opt, q->hhf_evict_timeout = usecs_to_jiffies(us); } - qlen = sch->q.qlen; - prev_backlog = sch->qstats.backlog; while (sch->q.qlen > sch->limit) { struct sk_buff *skb = qdisc_dequeue_internal(sch, false); + if (!skb) + break; + + dropped_pkts++; + dropped_bytes += qdisc_pkt_len(skb); rtnl_kfree_skbs(skb, skb); } - qdisc_tree_reduce_backlog(sch, qlen - sch->q.qlen, - prev_backlog - sch->qstats.backlog); + qdisc_tree_reduce_backlog(sch, dropped_pkts, dropped_bytes); sch_tree_unlock(sch); return 0; diff --git a/net/sched/sch_pie.c b/net/sched/sch_pie.c index fba806f0acd47..9b41c68f9b97a 100644 --- a/net/sched/sch_pie.c +++ b/net/sched/sch_pie.c @@ -138,9 +138,9 @@ static const struct nla_policy pie_policy[TCA_PIE_MAX + 1] = { static int pie_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { + unsigned int dropped_pkts = 0, dropped_bytes = 0; struct pie_sched_data *q = qdisc_priv(sch); struct nlattr *tb[TCA_PIE_MAX + 1]; - unsigned int qlen, dropped = 0; int err; err = nla_parse_nested_deprecated(tb, TCA_PIE_MAX, opt, pie_policy, @@ -188,15 +188,17 @@ static int pie_change(struct Qdisc *sch, struct nlattr *opt, nla_get_u32(tb[TCA_PIE_DQ_RATE_ESTIMATOR]); /* Drop excess packets if new limit is lower */ - qlen = sch->q.qlen; while (sch->q.qlen > sch->limit) { struct sk_buff *skb = qdisc_dequeue_internal(sch, true); - dropped += qdisc_pkt_len(skb); - qdisc_qstats_backlog_dec(sch, skb); + if (!skb) + break; + + dropped_pkts++; + dropped_bytes += qdisc_pkt_len(skb); rtnl_qdisc_drop(skb, sch); } - qdisc_tree_reduce_backlog(sch, qlen - sch->q.qlen, dropped); + qdisc_tree_reduce_backlog(sch, dropped_pkts, dropped_bytes); sch_tree_unlock(sch); return 0; -- 2.53.0