[PATCH v2 1/8] vsock/virtio: limit the memory used per-socket

[Stefano Garzarella]

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 avoiding to copy it.
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 solves this issue copying the payload in a new buffer.
Then it is queued in the per-socket list, and the 4KB buffer used
by the host is freed.

In this way, the memory used by each socket respects the credit
available, and we still avoid starvation, paying the cost of an
extra memory copy. When the buffer is completely full we do a
"zero-copy", moving the buffer directly in the per-socket list.

Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
---
 drivers/vhost/vsock.c                   |  2 +
 include/linux/virtio_vsock.h            |  8 +++
 net/vmw_vsock/virtio_transport.c        |  1 +
 net/vmw_vsock/virtio_transport_common.c | 95 ++++++++++++++++++-------
 4 files changed, 81 insertions(+), 25 deletions(-)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index bb5fc0e9fbc2..7964e2daee09 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -320,6 +320,8 @@ 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",
diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index e223e2632edd..345f04ee9193 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -54,9 +54,17 @@ struct virtio_vsock_pkt {
 	void *buf;
 	u32 len;
 	u32 off;
+	u32 buf_len;
 	bool reply;
 };
 
+struct virtio_vsock_buf {
+	struct list_head list;
+	void *addr;
+	u32 len;
+	u32 off;
+};
+
 struct virtio_vsock_pkt_info {
 	u32 remote_cid, remote_port;
 	struct vsock_sock *vsk;
diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index 15eb5d3d4750..af1d2ce12f54 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -280,6 +280,7 @@ 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));
diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 602715fc9a75..0248d6808755 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -65,6 +65,9 @@ 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;
@@ -86,6 +89,46 @@ virtio_transport_alloc_pkt(struct virtio_vsock_pkt_info *info,
 	return NULL;
 }
 
+static struct virtio_vsock_buf *
+virtio_transport_alloc_buf(struct virtio_vsock_pkt *pkt, bool zero_copy)
+{
+	struct virtio_vsock_buf *buf;
+
+	if (pkt->len == 0)
+		return NULL;
+
+	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
+	if (!buf)
+		return NULL;
+
+	/* If the buffer in the virtio_vsock_pkt is full, we can move it to
+	 * the new virtio_vsock_buf avoiding the copy, because we are sure that
+	 * we are not use more memory than that counted by the credit mechanism.
+	 */
+	if (zero_copy && pkt->len == pkt->buf_len) {
+		buf->addr = pkt->buf;
+		pkt->buf = NULL;
+	} else {
+		buf->addr = kmalloc(pkt->len, GFP_KERNEL);
+		if (!buf->addr) {
+			kfree(buf);
+			return NULL;
+		}
+
+		memcpy(buf->addr, pkt->buf, pkt->len);
+	}
+
+	buf->len = pkt->len;
+
+	return buf;
+}
+
+static void virtio_transport_free_buf(struct virtio_vsock_buf *buf)
+{
+	kfree(buf->addr);
+	kfree(buf);
+}
+
 /* Packet capture */
 static struct sk_buff *virtio_transport_build_skb(void *opaque)
 {
@@ -190,17 +233,15 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
 	return virtio_transport_get_ops()->send_pkt(pkt);
 }
 
-static void virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
-					struct virtio_vsock_pkt *pkt)
+static void virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs, u32 len)
 {
-	vvs->rx_bytes += pkt->len;
+	vvs->rx_bytes += len;
 }
 
-static void virtio_transport_dec_rx_pkt(struct virtio_vsock_sock *vvs,
-					struct virtio_vsock_pkt *pkt)
+static void virtio_transport_dec_rx_pkt(struct virtio_vsock_sock *vvs, u32 len)
 {
-	vvs->rx_bytes -= pkt->len;
-	vvs->fwd_cnt += pkt->len;
+	vvs->rx_bytes -= len;
+	vvs->fwd_cnt += len;
 }
 
 void virtio_transport_inc_tx_pkt(struct virtio_vsock_sock *vvs, struct virtio_vsock_pkt *pkt)
@@ -254,36 +295,36 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
 				   size_t len)
 {
 	struct virtio_vsock_sock *vvs = vsk->trans;
-	struct virtio_vsock_pkt *pkt;
+	struct virtio_vsock_buf *buf;
 	size_t bytes, total = 0;
 	int err = -EFAULT;
 
 	spin_lock_bh(&vvs->rx_lock);
 	while (total < len && !list_empty(&vvs->rx_queue)) {
-		pkt = list_first_entry(&vvs->rx_queue,
-				       struct virtio_vsock_pkt, list);
+		buf = list_first_entry(&vvs->rx_queue,
+				       struct virtio_vsock_buf, list);
 
 		bytes = len - total;
-		if (bytes > pkt->len - pkt->off)
-			bytes = pkt->len - pkt->off;
+		if (bytes > buf->len - buf->off)
+			bytes = buf->len - buf->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, pkt->buf + pkt->off, bytes);
+		err = memcpy_to_msg(msg, buf->addr + buf->off, bytes);
 		if (err)
 			goto out;
 
 		spin_lock_bh(&vvs->rx_lock);
 
 		total += bytes;
-		pkt->off += bytes;
-		if (pkt->off == pkt->len) {
-			virtio_transport_dec_rx_pkt(vvs, pkt);
-			list_del(&pkt->list);
-			virtio_transport_free_pkt(pkt);
+		buf->off += bytes;
+		if (buf->off == buf->len) {
+			virtio_transport_dec_rx_pkt(vvs, buf->len);
+			list_del(&buf->list);
+			virtio_transport_free_buf(buf);
 		}
 	}
 	spin_unlock_bh(&vvs->rx_lock);
@@ -841,20 +882,24 @@ virtio_transport_recv_connected(struct sock *sk,
 {
 	struct vsock_sock *vsk = vsock_sk(sk);
 	struct virtio_vsock_sock *vvs = vsk->trans;
+	struct virtio_vsock_buf *buf;
 	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;
+		buf = virtio_transport_alloc_buf(pkt, true);
 
-		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);
+		if (buf) {
+			spin_lock_bh(&vvs->rx_lock);
+			virtio_transport_inc_rx_pkt(vvs, pkt->len);
+			list_add_tail(&buf->list, &vvs->rx_queue);
+			spin_unlock_bh(&vvs->rx_lock);
 
-		sk->sk_data_ready(sk);
-		return err;
+			sk->sk_data_ready(sk);
+		}
+
+		break;
 	case VIRTIO_VSOCK_OP_CREDIT_UPDATE:
 		sk->sk_write_space(sk);
 		break;
-- 
2.20.1