Re: [PATCH v2 net 7/7] virtio_net: Differentiate sk_buff and xdp_frame on freeing

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 29, 2019 at 09:45:59AM +0900, Toshiaki Makita wrote:
> We do not reset or free up unused buffers when enabling/disabling XDP,
> so it can happen that xdp_frames are freed after disabling XDP or
> sk_buffs are freed after enabling XDP on xdp tx queues.
> Thus we need to handle both forms (xdp_frames and sk_buffs) regardless
> of XDP setting.
> One way to trigger this problem is to disable XDP when napi_tx is
> enabled. In that case, virtnet_xdp_set() calls virtnet_napi_enable()
> which kicks NAPI. The NAPI handler will call virtnet_poll_cleantx()
> which invokes free_old_xmit_skbs() for queues which have been used by
> XDP.
> 
> Note that even with this change we need to keep skipping
> free_old_xmit_skbs() from NAPI handlers when XDP is enabled, because XDP
> tx queues do not aquire queue locks.
> 
> - v2: Use napi_consume_skb() instead of dev_consume_skb_any()
> 
> Fixes: 4941d472bf95 ("virtio-net: do not reset during XDP set")
> Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> NOTE: Dropped Acked-by because of the v2 change.
> 
>  drivers/net/virtio_net.c | 54 +++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 42 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 1d454ce..2594481 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -57,6 +57,8 @@
>  #define VIRTIO_XDP_TX		BIT(0)
>  #define VIRTIO_XDP_REDIR	BIT(1)
>  
> +#define VIRTIO_XDP_FLAG	BIT(0)
> +
>  /* RX packet size EWMA. The average packet size is used to determine the packet
>   * buffer size when refilling RX rings. As the entire RX ring may be refilled
>   * at once, the weight is chosen so that the EWMA will be insensitive to short-
> @@ -252,6 +254,21 @@ struct padded_vnet_hdr {
>  	char padding[4];
>  };
>  
> +static bool is_xdp_frame(void *ptr)
> +{
> +	return (unsigned long)ptr & VIRTIO_XDP_FLAG;
> +}
> +
> +static void *xdp_to_ptr(struct xdp_frame *ptr)
> +{
> +	return (void *)((unsigned long)ptr | VIRTIO_XDP_FLAG);
> +}
> +
> +static struct xdp_frame *ptr_to_xdp(void *ptr)
> +{
> +	return (struct xdp_frame *)((unsigned long)ptr & ~VIRTIO_XDP_FLAG);
> +}
> +
>  /* Converting between virtqueue no. and kernel tx/rx queue no.
>   * 0:rx0 1:tx0 2:rx1 3:tx1 ... 2N:rxN 2N+1:txN 2N+2:cvq
>   */
> @@ -462,7 +479,8 @@ static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
>  
>  	sg_init_one(sq->sg, xdpf->data, xdpf->len);
>  
> -	err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdpf, GFP_ATOMIC);
> +	err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdp_to_ptr(xdpf),
> +				   GFP_ATOMIC);
>  	if (unlikely(err))
>  		return -ENOSPC; /* Caller handle free/refcnt */
>  
> @@ -482,13 +500,13 @@ static int virtnet_xdp_xmit(struct net_device *dev,
>  {
>  	struct virtnet_info *vi = netdev_priv(dev);
>  	struct receive_queue *rq = vi->rq;
> -	struct xdp_frame *xdpf_sent;
>  	struct bpf_prog *xdp_prog;
>  	struct send_queue *sq;
>  	unsigned int len;
>  	int drops = 0;
>  	int kicks = 0;
>  	int ret, err;
> +	void *ptr;
>  	int i;
>  
>  	/* Only allow ndo_xdp_xmit if XDP is loaded on dev, as this
> @@ -507,8 +525,12 @@ static int virtnet_xdp_xmit(struct net_device *dev,
>  	}
>  
>  	/* Free up any pending old buffers before queueing new ones. */
> -	while ((xdpf_sent = virtqueue_get_buf(sq->vq, &len)) != NULL)
> -		xdp_return_frame(xdpf_sent);
> +	while ((ptr = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> +		if (likely(is_xdp_frame(ptr)))
> +			xdp_return_frame(ptr_to_xdp(ptr));
> +		else
> +			napi_consume_skb(ptr, false);
> +	}
>  
>  	for (i = 0; i < n; i++) {
>  		struct xdp_frame *xdpf = frames[i];
> @@ -1329,18 +1351,26 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
>  
>  static void free_old_xmit_skbs(struct send_queue *sq, bool in_napi)
>  {
> -	struct sk_buff *skb;
>  	unsigned int len;
>  	unsigned int packets = 0;
>  	unsigned int bytes = 0;
> +	void *ptr;
>  
> -	while ((skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> -		pr_debug("Sent skb %p\n", skb);
> +	while ((ptr = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> +		if (likely(!is_xdp_frame(ptr))) {
> +			struct sk_buff *skb = ptr;
>  
> -		bytes += skb->len;
> -		packets++;
> +			pr_debug("Sent skb %p\n", skb);
>  
> -		napi_consume_skb(skb, in_napi);
> +			bytes += skb->len;
> +			napi_consume_skb(skb, in_napi);
> +		} else {
> +			struct xdp_frame *frame = ptr_to_xdp(ptr);
> +
> +			bytes += frame->len;
> +			xdp_return_frame(frame);
> +		}
> +		packets++;
>  	}
>  
>  	/* Avoid overhead when no packets have been processed
> @@ -2666,10 +2696,10 @@ static void free_unused_bufs(struct virtnet_info *vi)
>  	for (i = 0; i < vi->max_queue_pairs; i++) {
>  		struct virtqueue *vq = vi->sq[i].vq;
>  		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL) {
> -			if (!is_xdp_raw_buffer_queue(vi, i))
> +			if (!is_xdp_frame(buf))
>  				dev_kfree_skb(buf);
>  			else
> -				xdp_return_frame(buf);
> +				xdp_return_frame(ptr_to_xdp(buf));
>  		}
>  	}
>  
> -- 
> 1.8.3.1
>