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

From: Jason Wang
Date: Fri Jan 17 2014 - 04:42:46 EST

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@xxxxxxxxxx>
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
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))
+ if (!(q->flags & TCQ_F_CAN_BYPASS) &&
+ unlikely(skb_orphan_frags(skb, GFP_ATOMIC))) {
+ kfree_skb(skb);
+ goto out;
+ }

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,
if (unlikely(contended))
return rc;

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