Re: [PATCH bpf] net, xsk: Avoid taking multiple skbuff references

From: Björn Töpel <redacted>

Commit 642e450b6b59 ("xsk: Do not discard packet when NETDEV_TX_BUSY")
addressed the problem that packets were discarded from the Tx AF_XDP
ring, when the driver returned NETDEV_TX_BUSY. Part of the fix was
bumping the skbuff reference count, so that the buffer would not be
freed by dev_direct_xmit(). A reference count larger than one means
that the skbuff is "shared", which is not the case.

If the "shared" skbuff is sent to the generic XDP receive path,
netif_receive_generic_xdp(), and pskb_expand_head() is entered the
BUG_ON(skb_shared(skb)) will trigger.

This patch adds a variant to dev_direct_xmit(), __dev_direct_xmit(),
where a user can select the skbuff free policy. This allows AF_XDP to
avoid bumping the reference count, but still keep the NETDEV_TX_BUSY
behavior.

Reported-by: Yonghong Song <redacted>
Fixes: 642e450b6b59 ("xsk: Do not discard packet when NETDEV_TX_BUSY")
Signed-off-by: Björn Töpel <redacted>
---
  include/linux/netdevice.h | 1 +
  net/core/dev.c            | 9 +++++++--
  net/xdp/xsk.c             | 8 +-------
  3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 964b494b0e8d..e7402fca7752 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h

@@ -2815,6 +2815,7 @@ u16 dev_pick_tx_cpu_id(struct net_device *dev, struct sk_buff *skb,
  		       struct net_device *sb_dev);
  int dev_queue_xmit(struct sk_buff *skb);
  int dev_queue_xmit_accel(struct sk_buff *skb, struct net_device *sb_dev);
+int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id, bool free_on_busy);
  int dev_direct_xmit(struct sk_buff *skb, u16 queue_id);
  int register_netdevice(struct net_device *dev);
  void unregister_netdevice_queue(struct net_device *dev, struct list_head *head);

diff --git a/net/core/dev.c b/net/core/dev.c
index 82dc6b48e45f..2af79a4253bb 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -4180,7 +4180,7 @@ int dev_queue_xmit_accel(struct sk_buff *skb, struct net_device *sb_dev)
  }
  EXPORT_SYMBOL(dev_queue_xmit_accel);
  
-int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
+int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id, bool free_on_busy)
  {
  	struct net_device *dev = skb->dev;
  	struct sk_buff *orig_skb = skb;

@@ -4211,7 +4211,7 @@ int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
  
  	local_bh_enable();
  
-	if (!dev_xmit_complete(ret))
+	if (free_on_busy && !dev_xmit_complete(ret))
  		kfree_skb(skb);
  
  	return ret;

@@ -4220,6 +4220,11 @@ int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
  	kfree_skb_list(skb);
  	return NET_XMIT_DROP;
  }
+
+int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
+{
+	return __dev_direct_xmit(skb, queue_id, true);
+}
  EXPORT_SYMBOL(dev_direct_xmit);
  
  /*************************************************************************

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 5a6cdf7b320d..c6ad31b374b7 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c

@@ -411,11 +411,7 @@ static int xsk_generic_xmit(struct sock *sk)
  		skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;
  		skb->destructor = xsk_destruct_skb;
  
-		/* Hinder dev_direct_xmit from freeing the packet and
-		 * therefore completing it in the destructor
-		 */
-		refcount_inc(&skb->users);
-		err = dev_direct_xmit(skb, xs->queue_id);
+		err = __dev_direct_xmit(skb, xs->queue_id, false);
  		if  (err == NETDEV_TX_BUSY) {
  			/* Tell user-space to retry the send */
  			skb->destructor = sock_wfree;

@@ -429,12 +425,10 @@ static int xsk_generic_xmit(struct sock *sk)
  		/* Ignore NET_XMIT_CN as packet might have been sent */
  		if (err == NET_XMIT_DROP) {
  			/* SKB completed but not sent */
-			kfree_skb(skb);
  			err = -EBUSY;
  			goto out;
  		}
  
-		consume_skb(skb);
  		sent_frame = true;
  	}
  

base-commit: 178648916e73e00de83150eb0c90c0d3a977a46a