linux-kernel - [PATCH] tap/tun: add stats accounting when failed to transfer data to user

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Message-Id: <20250730092940.1439637-1-zhangyanjun@cestc.cn>
Date: Wed, 30 Jul 2025 17:29:40 +0800
From: zhangyanjun@...tc.cn
To: willemdebruijn.kernel@...il.com,
	jasowang@...hat.com
Cc: netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	zhangyanjun@...tc.cn
Subject: [PATCH] tap/tun: add stats accounting when failed to transfer data to user

From: Yanjun Zhang <zhangyanjun@...tc.cn>

To more accurately detect packet dropped, we add the dropped packet
counter with the device when kfree_skb is called because of failing
to transfer data to user space.

Signed-off-by: Yanjun Zhang <zhangyanjun@...tc.cn>
---
 drivers/net/tap.c | 14 +++++++++++---
 drivers/net/tun.c |  9 ++++++---
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index bdf0788d8..9d288a1ad 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -759,6 +759,8 @@ static ssize_t tap_do_read(struct tap_queue *q,
 {
 	DEFINE_WAIT(wait);
 	ssize_t ret = 0;
+	struct tap_dev *tap;
+	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 
 	if (!iov_iter_count(to)) {
 		kfree_skb(skb);
@@ -794,10 +796,16 @@ static ssize_t tap_do_read(struct tap_queue *q,
 put:
 	if (skb) {
 		ret = tap_put_user(q, skb, to);
-		if (unlikely(ret < 0))
-			kfree_skb(skb);
-		else
+		if (unlikely(ret < 0)) {
+			kfree_skb_reason(skb, drop_reason);
+			rcu_read_lock();
+			tap = rcu_dereference(q->tap);
+			if (tap && tap->count_rx_dropped)
+				tap->count_rx_dropped(tap);
+			rcu_read_unlock();
+		} else {
 			consume_skb(skb);
+		}
 	}
 	return ret;
 }
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index f8c5e2fd0..eb3c68e5f 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2137,6 +2137,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
 {
 	ssize_t ret;
 	int err;
+	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 
 	if (!iov_iter_count(to)) {
 		tun_ptr_free(ptr);
@@ -2159,10 +2160,12 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
 		struct sk_buff *skb = ptr;
 
 		ret = tun_put_user(tun, tfile, skb, to);
-		if (unlikely(ret < 0))
-			kfree_skb(skb);
-		else
+		if (unlikely(ret < 0)) {
+			dev_core_stats_tx_dropped_inc(tun->dev);
+			kfree_skb_reason(skb, drop_reason);
+		} else {
 			consume_skb(skb);
+		}
 	}
 
 	return ret;
-- 
2.31.1