Subject: Re: [PATCH v1] vsock/virtio: rework MSG_ZEROCOPY flag handling

From: David Laight <[email-masked]>
Date: Fri, 5 Jun 2026 16:08:51 +0100

On Fri,  5 Jun 2026 14:53:14 +0300
Arseniy Krasnov <[email-masked]> wrote:

> Logically it was based on TCP implementation, so to make further
> support easier, rewrite it in the TCP way.
> 
> Signed-off-by: Arseniy Krasnov <[email-masked]>
> ---
>  net/vmw_vsock/virtio_transport_common.c | 64 ++++++++++++-------------
>  1 file changed, 32 insertions(+), 32 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 2fd9eaaf5ca6..00caeeaa5590 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -73,10 +73,13 @@ static bool virtio_transport_can_zcopy(const struct virtio_transport *t_ops,
>  static int virtio_transport_fill_skb(struct sk_buff *skb,
>  				     struct virtio_vsock_pkt_info *info,
>  				     size_t len,
> -				     bool zcopy)
> +				     bool zcopy, struct ubuf_info *uarg)
>  {
>  	struct msghdr *msg = info->msg;
>  
> +	/* We have completion - attach it to 'skb'. */
> +	skb_zcopy_set(skb, uarg, NULL);
> +
>  	if (zcopy)
>  		return __zerocopy_sg_from_iter(msg, NULL, skb,
>  					       &msg->msg_iter, len, NULL);
> @@ -208,7 +211,8 @@ static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *
>  						  u32 src_cid,
>  						  u32 src_port,
>  						  u32 dst_cid,
> -						  u32 dst_port)
> +						  u32 dst_port,
> +						  struct ubuf_info *uarg)
>  {
>  	struct vsock_sock *vsk;
>  	struct sk_buff *skb;
> @@ -245,7 +249,7 @@ static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *
>  	if (info->msg && payload_len > 0) {
>  		int err;
>  
> -		err = virtio_transport_fill_skb(skb, info, payload_len, zcopy);
> +		err = virtio_transport_fill_skb(skb, info, payload_len, zcopy, uarg);
>  		if (err)
>  			goto out;
>  
> @@ -321,38 +325,36 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>  	if (pkt_len == 0 && info->op == VIRTIO_VSOCK_OP_RW)
>  		return pkt_len;
>  
> -	if (info->msg) {
> -		/* If zerocopy is not enabled by 'setsockopt()', we behave as
> -		 * there is no MSG_ZEROCOPY flag set.
> +	if (info->msg && (info->msg->msg_flags & MSG_ZEROCOPY)) {
> +		/* If 'info->msg' is not NULL, this is only VIRTIO_VSOCK_OP_RW.
> +		 * 'MSG_ZEROCOPY' flag handling here is based on the same flag
> +		 * handling from 'tcp_sendmsg_locked()'.
>  		 */
> -		if (!sock_flag(sk_vsock(vsk), SOCK_ZEROCOPY))
> -			info->msg->msg_flags &= ~MSG_ZEROCOPY;
> +		if (info->msg->msg_ubuf) {
> +			uarg = info->msg->msg_ubuf;
> +			can_zcopy = virtio_transport_can_zcopy(t_ops, info, pkt_len);
> +		} else if (sock_flag(sk_vsock(vsk), SOCK_ZEROCOPY)) {
> +			uarg = msg_zerocopy_realloc(sk_vsock(vsk), pkt_len,
> +						    NULL, false);
> +			if (!uarg) {
> +				virtio_transport_put_credit(vvs, pkt_len);
> +				return -ENOMEM;
> +			}
>  
> -		if (info->msg->msg_flags & MSG_ZEROCOPY)
>  			can_zcopy = virtio_transport_can_zcopy(t_ops, info, pkt_len);
>  
> +			if (!can_zcopy)
> +				uarg_to_msgzc(uarg)->zerocopy = 0;
> +
> +			have_uref = true;
> +		}
> +
> +		/* 'can_zcopy' means that this transmission will be
> +		 * in zerocopy way (e.g. using 'frags' array).
> +		 */

I've not looked at the tcp code, but the above doesn't look right.
I don't see why msg->msg_ubuf might be non-NULL without SOCK_ZEROCOPY set.
That would give the outer code a callback when the last skb is freed but
still copy the data.

I also don't see the point of calling msg_zerocopy_realloc() to get a
callback when the last skb is freed and then setting
	uarg_to_msgzc(uarg)->zerocopy = 0;
so that the callback doesn't actually do anything.
It isn't as though you 'find out' later on that you can't actually do
zerocopy.

>  		if (can_zcopy)
>  			max_skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE,
>  					    (MAX_SKB_FRAGS * PAGE_SIZE));
> -
> -		if (info->msg->msg_flags & MSG_ZEROCOPY &&
> -		    info->op == VIRTIO_VSOCK_OP_RW) {
> -			uarg = info->msg->msg_ubuf;
> -
> -			if (!uarg) {
> -				uarg = msg_zerocopy_realloc(sk_vsock(vsk),
> -							    pkt_len, NULL, false);
> -				if (!uarg) {
> -					virtio_transport_put_credit(vvs, pkt_len);
> -					return -ENOMEM;
> -				}
> -
> -				if (!can_zcopy)
> -					uarg_to_msgzc(uarg)->zerocopy = 0;
> -
> -				have_uref = true;
> -			}
> -		}
>  	}
>  
>  	rest_len = pkt_len;
> @@ -365,14 +367,12 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>  
>  		skb = virtio_transport_alloc_skb(info, skb_len, can_zcopy,
>  						 src_cid, src_port,
> -						 dst_cid, dst_port);
> +						 dst_cid, dst_port, uarg);
>  		if (!skb) {
>  			ret = -ENOMEM;
>  			break;
>  		}
>  
> -		skb_zcopy_set(skb, uarg, NULL);

Aren't you passing uarg through two function calls instead of doing it here.
Doesn't even make it clearer what is going on.

-- David

> -
>  		virtio_transport_inc_tx_pkt(vvs, skb);
>  
>  		ret = t_ops->send_pkt(skb, info->net);
> @@ -1178,7 +1178,7 @@ static int virtio_transport_reset_no_sock(const struct virtio_transport *t,
>  					   le64_to_cpu(hdr->dst_cid),
>  					   le32_to_cpu(hdr->dst_port),
>  					   le64_to_cpu(hdr->src_cid),
> -					   le32_to_cpu(hdr->src_port));
> +					   le32_to_cpu(hdr->src_port), NULL);
>  	if (!reply)
>  		return -ENOMEM;
>