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

[Stefano Garzarella <sgarzare@redhat.com>]

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@sberdevices.ru>
>---
> 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;

What about storing `VIRTIO_VSOCK_SKB_CB(skb)->frag_off` in a variable?
More for readability than optimization, which I hope the compiler
already does on its own.

The rest LGTM.

Stefano

>
> 		/* 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
>