[PATCH net-next v1 1/4] vsock/virtio/vhost: read data from non-linear skb

[Arseniy Krasnov]

This is preparation patch for MSG_ZEROCOPY support. It adds handling of
non-linear skbs by replacing direct calls of 'memcpy_to_msg()' with
'skb_copy_datagram_iter()'. Main advantage of the second one is that it
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 behind this field 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>
---
 Changelog:
 v5(big patchset) -> v1:
  * Merge 'virtio_transport_common.c' and 'vhost/vsock.c' patches into
    this single patch.
  * Commit message update: grammar fix and remark that this patch is
    MSG_ZEROCOPY preparation.
  * Use 'min_t()' instead of comparison using '<>' operators.

 drivers/vhost/vsock.c                   | 14 ++++++++-----
 include/linux/virtio_vsock.h            |  1 +
 net/vmw_vsock/virtio_transport_common.c | 27 ++++++++++++++++---------
 3 files changed, 28 insertions(+), 14 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 817d377a3f36..8c917be32b5d 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -114,6 +114,7 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 		struct sk_buff *skb;
 		unsigned out, in;
 		size_t nbytes;
+		u32 frag_off;
 		int head;
 
 		skb = virtio_vsock_skb_dequeue(&vsock->send_pkt_queue);
@@ -156,7 +157,8 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 		}
 
 		iov_iter_init(&iov_iter, ITER_DEST, &vq->iov[out], in, iov_len);
-		payload_len = skb->len;
+		frag_off = VIRTIO_VSOCK_SKB_CB(skb)->frag_off;
+		payload_len = skb->len - frag_off;
 		hdr = virtio_vsock_hdr(skb);
 
 		/* If the packet is greater than the space available in the
@@ -197,8 +199,10 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 			break;
 		}
 
-		nbytes = copy_to_iter(skb->data, payload_len, &iov_iter);
-		if (nbytes != payload_len) {
+		if (skb_copy_datagram_iter(skb,
+					   frag_off,
+					   &iov_iter,
+					   payload_len)) {
 			kfree_skb(skb);
 			vq_err(vq, "Faulted on copying pkt buf\n");
 			break;
@@ -212,13 +216,13 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
 		vhost_add_used(vq, head, sizeof(*hdr) + payload_len);
 		added = true;
 
-		skb_pull(skb, payload_len);
+		VIRTIO_VSOCK_SKB_CB(skb)->frag_off += payload_len;
 		total_len += payload_len;
 
 		/* If we didn't send all the payload we can requeue the packet
 		 * to send it with the next available buffer.
 		 */
-		if (skb->len > 0) {
+		if (VIRTIO_VSOCK_SKB_CB(skb)->frag_off < skb->len) {
 			hdr->flags |= cpu_to_le32(flags_to_restore);
 
 			/* We are queueing the same skb to handle
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..1a376f808ae6 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;
 
@@ -413,25 +416,28 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
 	while (total < len && !skb_queue_empty(&vvs->rx_queue)) {
 		skb = skb_peek(&vvs->rx_queue);
 
-		bytes = len - total;
-		if (bytes > skb->len)
-			bytes = skb->len;
+		bytes = min_t(size_t, len - total,
+			      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 +509,10 @@ 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