vsock/virtio: limit the memory used per-socket

Since virtio-vsock was introduced, the buffers filled by the host
and pushed to the guest using the vring, are directly queued in
a per-socket list. These buffers are preallocated by the guest
with a fixed size (4 KB).

The maximum amount of memory used by each socket should be
controlled by the credit mechanism.
The default credit available per-socket is 256 KB, but if we use
only 1 byte per packet, the guest can queue up to 262144 of 4 KB
buffers, using up to 1 GB of memory per-socket. In addition, the
guest will continue to fill the vring with new 4 KB free buffers
to avoid starvation of other sockets.

This patch mitigates this issue copying the payload of small
packets (< 128 bytes) into the buffer of last packet queued, in
order to avoid wasting memory.
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/vsock.c

@@ -329,6 +329,8 @@ vhost_vsock_alloc_pkt(struct vhost_virtqueue *vq,
 		return NULL;
 	}
 
+	pkt->buf_len = pkt->len;
+
 	nbytes = copy_from_iter(pkt->buf, pkt->len, &iov_iter);
 	if (nbytes != pkt->len) {
 		vq_err(vq, "Expected %u byte payload, got %zu bytes\n",

include/linux/virtio_vsock.h

@@ -52,6 +52,7 @@ struct virtio_vsock_pkt {
 	/* socket refcnt not held, only use for cancellation */
 	struct vsock_sock *vsk;
 	void *buf;
+	u32 buf_len;
 	u32 len;
 	u32 off;
 	bool reply;

net/vmw_vsock/virtio_transport.c

@@ -307,6 +307,7 @@ static void virtio_vsock_rx_fill(struct virtio_vsock *vsock)
 			break;
 		}
 
+		pkt->buf_len = buf_len;
 		pkt->len = buf_len;
 
 		sg_init_one(&hdr, &pkt->hdr, sizeof(pkt->hdr));

net/vmw_vsock/virtio_transport_common.c

@@ -26,6 +26,9 @@
 /* How long to wait for graceful shutdown of a connection */
 #define VSOCK_CLOSE_TIMEOUT (8 * HZ)
 
+/* Threshold for detecting small packets to copy */
+#define GOOD_COPY_LEN  128
+
 static const struct virtio_transport *virtio_transport_get_ops(void)
 {
 	const struct vsock_transport *t = vsock_core_get_transport();
@@ -64,6 +67,9 @@ virtio_transport_alloc_pkt(struct virtio_vsock_pkt_info *info,
 		pkt->buf = kmalloc(len, GFP_KERNEL);
 		if (!pkt->buf)
 			goto out_pkt;
+
+		pkt->buf_len = len;
+
 		err = memcpy_from_msg(pkt->buf, info->msg, len);
 		if (err)
 			goto out;
@@ -841,24 +847,60 @@ virtio_transport_recv_connecting(struct sock *sk,
 	return err;
 }
 
+static void
+virtio_transport_recv_enqueue(struct vsock_sock *vsk,
+			      struct virtio_vsock_pkt *pkt)
+{
+	struct virtio_vsock_sock *vvs = vsk->trans;
+	bool free_pkt = false;
+
+	pkt->len = le32_to_cpu(pkt->hdr.len);
+	pkt->off = 0;
+
+	spin_lock_bh(&vvs->rx_lock);
+
+	virtio_transport_inc_rx_pkt(vvs, pkt);
+
+	/* Try to copy small packets into the buffer of last packet queued,
+	 * to avoid wasting memory queueing the entire buffer with a small
+	 * payload.
+	 */
+	if (pkt->len <= GOOD_COPY_LEN && !list_empty(&vvs->rx_queue)) {
+		struct virtio_vsock_pkt *last_pkt;
+
+		last_pkt = list_last_entry(&vvs->rx_queue,
+					   struct virtio_vsock_pkt, list);
+
+		/* If there is space in the last packet queued, we copy the
+		 * new packet in its buffer.
+		 */
+		if (pkt->len <= last_pkt->buf_len - last_pkt->len) {
+			memcpy(last_pkt->buf + last_pkt->len, pkt->buf,
+			       pkt->len);
+			last_pkt->len += pkt->len;
+			free_pkt = true;
+			goto out;
+		}
+	}
+
+	list_add_tail(&pkt->list, &vvs->rx_queue);
+
+out:
+	spin_unlock_bh(&vvs->rx_lock);
+	if (free_pkt)
+		virtio_transport_free_pkt(pkt);
+}
+
 static int
 virtio_transport_recv_connected(struct sock *sk,
 				struct virtio_vsock_pkt *pkt)
 {
 	struct vsock_sock *vsk = vsock_sk(sk);
-	struct virtio_vsock_sock *vvs = vsk->trans;
 	int err = 0;
 
 	switch (le16_to_cpu(pkt->hdr.op)) {
 	case VIRTIO_VSOCK_OP_RW:
-		pkt->len = le32_to_cpu(pkt->hdr.len);
-		pkt->off = 0;
-
-		spin_lock_bh(&vvs->rx_lock);
-		virtio_transport_inc_rx_pkt(vvs, pkt);
-		list_add_tail(&pkt->list, &vvs->rx_queue);
-		spin_unlock_bh(&vvs->rx_lock);
-
+		virtio_transport_recv_enqueue(vsk, pkt);
 		sk->sk_data_ready(sk);
 		return err;
 	case VIRTIO_VSOCK_OP_CREDIT_UPDATE: