[PATCH net] net: core: orphan frags before queuing to slow qdisc

Many qdiscs can queue a packet for a long time, this will lead an issue
with zerocopy skb. It means the frags will not be orphaned in an expected
short time, this breaks the assumption that virtio-net will transmit the
packet in time.

So if guest packets were queued through such kind of qdisc and hit the
limitation of the max pending packets for virtio/vhost. All packets that
go to another destination from guest will also be blocked.

A case for reproducing the issue:

- Boot two VMs and connect them to the same bridge kvmbr.
- Setup tbf with a very low rate/burst on eth0 which is a port of kvmbr.
- Let VM1 send lots of packets thorugh eth0
- After a while, VM1 is unable to send any packets out since the number of
  pending packets (queued to tbf) were exceeds the limitation of vhost/virito

Solve this issue by orphaning the frags before queuing it to a slow qdisc (the
one without TCQ_F_CAN_BYPASS).

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <redacted>
---
 net/core/dev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 0ce469e..1209774 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -2700,6 +2700,12 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 	contended = qdisc_is_running(q);
 	if (unlikely(contended))
 		spin_lock(&q->busylock);
+	if (!(q->flags & TCQ_F_CAN_BYPASS) &&
+	    unlikely(skb_orphan_frags(skb, GFP_ATOMIC))) {
+		kfree_skb(skb);
+		rc = NET_XMIT_DROP;
+		goto out;
+	}
 
 	spin_lock(root_lock);
 	if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {

@@ -2739,6 +2745,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 		}
 	}
 	spin_unlock(root_lock);
+out:
 	if (unlikely(contended))
 		spin_unlock(&q->busylock);
 	return rc;

-- 
1.8.3.2