On 11/23/20 2:12 PM, Björn Töpel wrote:
From: Björn Töpel <[email protected]>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 <[email protected]> Fixes: 642e450b6b59 ("xsk: Do not discard packet when NETDEV_TX_BUSY") Signed-off-by: Björn Töpel <[email protected]> --- 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;
Hm, but this way free_on_busy, even though constant, cannot be optimized away? Can't you just move the dev_xmit_complete() check out into dev_direct_xmit() instead? That way you can just drop the bool, and the below dev_direct_xmit() should probably just become an __always_line function in netdevice.h so you avoid the double call.
@@ -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
