virtio_net: fix support for small rings

When ring size is small (<32 entries) making buffers smaller means a
full ring might not be able to hold enough buffers to fit a single large
packet.

Make sure a ring full of buffers is large enough to allow at least one
packet of max size.

Fixes: 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page frag allocators")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/net/virtio_net.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

On Wed, Mar 29, 2017 at 08:15:04PM +0300, Michael S. Tsirkin wrote:
> When ring size is small (<32 entries) making buffers smaller means a
> full ring might not be able to hold enough buffers to fit a single large
> packet.
> 
> Make sure a ring full of buffers is large enough to allow at least one
> packet of max size.
> 
> Fixes: 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page frag allocators")
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Oops posted a wrong version. Pls ignore will repost.

> ---
>  drivers/net/virtio_net.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 9dc31dc..f6a379d 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -98,6 +98,9 @@ struct receive_queue {
>  	/* RX: fragments + linear part + virtio header */
>  	struct scatterlist sg[MAX_SKB_FRAGS + 2];
>  
> +	/* Min single buffer size for mergeable buffers case. */
> +	unsigned int min_buf_len;
> +
>  	/* Name of this receive queue: input.$index */
>  	char name[40];
>  };
> @@ -894,13 +897,14 @@ static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
>  	return err;
>  }
>  
> -static unsigned int get_mergeable_buf_len(struct ewma_pkt_len *avg_pkt_len)
> +static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
> +					  struct ewma_pkt_len *avg_pkt_len)
>  {
>  	const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
>  	unsigned int len;
>  
>  	len = hdr_len + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
> -			GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
> +				rq->min_buf_len - hdr_len, PAGE_SIZE - hdr_len);
>  	return ALIGN(len, L1_CACHE_BYTES);
>  }
>  
> @@ -914,7 +918,7 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
>  	int err;
>  	unsigned int len, hole;
>  
> -	len = get_mergeable_buf_len(&rq->mrg_avg_pkt_len);
> +	len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len);
>  	if (unlikely(!skb_page_frag_refill(len + headroom, alloc_frag, gfp)))
>  		return -ENOMEM;
>  
> @@ -2086,6 +2090,21 @@ static void virtnet_del_vqs(struct virtnet_info *vi)
>  	virtnet_free_queues(vi);
>  }
>  
> +/* How large should a single buffer be so a queue full of these can fit at
> + * least one full packet?
> + * Logic below assumes the mergeable buffer header is used.
> + */
> +static unsigned int mergeable_min_buf_len(struct virtnet_info *vi, struct virtqueue *vq)
> +{
> +	const unsigned int hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> +	unsigned int rq_size = virtqueue_get_vring_size(vq);
> +	unsigned int packet_len = vi->big_packets ? IP_MAX_MTU : vi->dev->max_mtu;
> +	unsigned int buf_len = hdr_len + ETH_HLEN + VLAN_HLEN + packet_len;
> +	unsigned int min_buf_len = DIV_ROUND_UP(buf_len, rq_size);
> +
> +	return max(min_buf_len, hdr_len);
> +}
> +
>  static int virtnet_find_vqs(struct virtnet_info *vi)
>  {
>  	vq_callback_t **callbacks;
> @@ -2151,6 +2170,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  
>  	for (i = 0; i < vi->max_queue_pairs; i++) {
>  		vi->rq[i].vq = vqs[rxq2vq(i)];
> +		vi->rq[i].min_buf_len = mergeable_min_buf_len(vi, vi->rq[i].vq);
>  		vi->sq[i].vq = vqs[txq2vq(i)];
>  	}
>  
> @@ -2237,7 +2257,8 @@ static ssize_t mergeable_rx_buffer_size_show(struct netdev_rx_queue *queue,
>  
>  	BUG_ON(queue_index >= vi->max_queue_pairs);
>  	avg = &vi->rq[queue_index].mrg_avg_pkt_len;
> -	return sprintf(buf, "%u\n", get_mergeable_buf_len(avg));
> +	return sprintf(buf, "%u\n",
> +		       get_mergeable_buf_len(&vi->rq[queue_index], avg));
>  }
>  
>  static struct rx_queue_attribute mergeable_rx_buffer_size_attribute =
> -- 
> MST