vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers

When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
transmit data. Unfortunately, these are always linear allocations and
can therefore result in significant pressure on kmalloc() considering
that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
allocation for each packet.

Rework the vsock SKB allocation so that, for sizes with page order
greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
instead with the packet header in the SKB and the transmit data in the
fragments. Note that this affects both the vhost and virtio transports.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
Message-Id: <20250717090116.11987-10-will@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Author: willdeacon

net/vmw_vsock/virtio_transport_common.c

@@ -109,7 +109,8 @@ static int virtio_transport_fill_skb(struct sk_buff *skb,
 		return __zerocopy_sg_from_iter(info->msg, NULL, skb,
 					       &info->msg->msg_iter, len, NULL);
 
-	return memcpy_from_msg(skb_put(skb, len), info->msg, len);
+	virtio_vsock_skb_put(skb, len);
+	return skb_copy_datagram_from_iter(skb, 0, &info->msg->msg_iter, len);
 }
 
 static void virtio_transport_init_hdr(struct sk_buff *skb,
@@ -261,7 +262,7 @@ static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *
 	if (!zcopy)
 		skb_len += payload_len;
 
-	skb = virtio_vsock_alloc_linear_skb(skb_len, GFP_KERNEL);
+	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
 	if (!skb)
 		return NULL;