Re: [RFC PATCH v4 01/17] vsock/virtio: read data from non-linear skb

[Bobby Eshleman]

On Sat, Jun 03, 2023 at 11:49:23PM +0300, Arseniy Krasnov wrote:
> This is preparation patch for non-linear skbuff handling. It replaces
> direct calls of 'memcpy_to_msg()' with 'skb_copy_datagram_iter()'. Main
> advantage of the second one is that is can handle paged part of the skb
> by using 'kmap()' on each page, but if there are no pages in the skb,
> it behaves like simple copying to iov iterator. This patch also adds
> new field to the control block of skb - this value shows current offset
> in the skb to read next portion of data (it doesn't matter linear it or
> not). Idea is that 'skb_copy_datagram_iter()' handles both types of
> skb internally - it just needs an offset from which to copy data from
> the given skb. This offset is incremented on each read from skb. This
> approach allows to avoid special handling of non-linear skbs:
> 1) We can't call 'skb_pull()' on it, because it updates 'data' pointer.
> 2) We need to update 'data_len' also on each read from this skb.
> 
> Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
> ---
>  include/linux/virtio_vsock.h            |  1 +
>  net/vmw_vsock/virtio_transport_common.c | 26 +++++++++++++++++--------
>  2 files changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> index c58453699ee9..17dbb7176e37 100644
> --- a/include/linux/virtio_vsock.h
> +++ b/include/linux/virtio_vsock.h
> @@ -12,6 +12,7 @@
>  struct virtio_vsock_skb_cb {
>  	bool reply;
>  	bool tap_delivered;
> +	u32 frag_off;
>  };
>  
>  #define VIRTIO_VSOCK_SKB_CB(skb) ((struct virtio_vsock_skb_cb *)((skb)->cb))
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index b769fc258931..5819a9cd4515 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -355,7 +355,7 @@ virtio_transport_stream_do_peek(struct vsock_sock *vsk,
>  	spin_lock_bh(&vvs->rx_lock);
>  
>  	skb_queue_walk_safe(&vvs->rx_queue, skb,  tmp) {
> -		off = 0;
> +		off = VIRTIO_VSOCK_SKB_CB(skb)->frag_off;
>  
>  		if (total == len)
>  			break;
> @@ -370,7 +370,10 @@ virtio_transport_stream_do_peek(struct vsock_sock *vsk,
>  			 */
>  			spin_unlock_bh(&vvs->rx_lock);
>  
> -			err = memcpy_to_msg(msg, skb->data + off, bytes);
> +			err = skb_copy_datagram_iter(skb, off,
> +						     &msg->msg_iter,
> +						     bytes);
> +
>  			if (err)
>  				goto out;
>  
> @@ -414,24 +417,28 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>  		skb = skb_peek(&vvs->rx_queue);
>  
>  		bytes = len - total;
> -		if (bytes > skb->len)
> -			bytes = skb->len;
> +		if (bytes > skb->len - VIRTIO_VSOCK_SKB_CB(skb)->frag_off)
> +			bytes = skb->len - VIRTIO_VSOCK_SKB_CB(skb)->frag_off;
>  
>  		/* sk_lock is held by caller so no one else can dequeue.
>  		 * Unlock rx_lock since memcpy_to_msg() may sleep.
>  		 */
>  		spin_unlock_bh(&vvs->rx_lock);
>  
> -		err = memcpy_to_msg(msg, skb->data, bytes);
> +		err = skb_copy_datagram_iter(skb,
> +					     VIRTIO_VSOCK_SKB_CB(skb)->frag_off,
> +					     &msg->msg_iter, bytes);
> +
>  		if (err)
>  			goto out;
>  
>  		spin_lock_bh(&vvs->rx_lock);
>  
>  		total += bytes;
> -		skb_pull(skb, bytes);
>  
> -		if (skb->len == 0) {
> +		VIRTIO_VSOCK_SKB_CB(skb)->frag_off += bytes;
> +
> +		if (skb->len == VIRTIO_VSOCK_SKB_CB(skb)->frag_off) {
>  			u32 pkt_len = le32_to_cpu(virtio_vsock_hdr(skb)->len);
>  
>  			virtio_transport_dec_rx_pkt(vvs, pkt_len);
> @@ -503,7 +510,10 @@ static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
>  				 */
>  				spin_unlock_bh(&vvs->rx_lock);
>  
> -				err = memcpy_to_msg(msg, skb->data, bytes_to_copy);
> +				err = skb_copy_datagram_iter(skb, 0,
> +							     &msg->msg_iter,
> +							     bytes_to_copy);
> +
>  				if (err) {
>  					/* Copy of message failed. Rest of
>  					 * fragments will be freed without copy.
> -- 
> 2.25.1
> 

LGTM.

Reviewed-by: Bobby Eshleman <bobby.eshleman@xxxxxxxxxxxxx>