Linux Virtualization - Jul 2019 - [PATCH v4 1/5] vsock/virtio: limit the memory used per-socket

On Wed, Jul 17, 2019 at 01:30:26PM +0200, Stefano Garzarella
wrote:
> 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.
> 
> Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
This is good enough for net-next, but for net I think we
should figure out how to address the issue completely.
Can we make the accounting precise? What happens to
performance if we do?

> ---
>  drivers/vhost/vsock.c                   |  2 +
>  include/linux/virtio_vsock.h            |  1 +
>  net/vmw_vsock/virtio_transport.c        |  1 +
>  net/vmw_vsock/virtio_transport_common.c | 60 +++++++++++++++++++++----
>  4 files changed, 55 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 6a50e1d0529c..6c8390a2af52 100644
> --- a/drivers/vhost/vsock.c
> +++ b/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",
> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> index e223e2632edd..7d973903f52e 100644
> --- a/include/linux/virtio_vsock.h
> +++ b/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;
> diff --git a/net/vmw_vsock/virtio_transport.c
b/net/vmw_vsock/virtio_transport.c
> index 0815d1357861..082a30936690 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/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));
> diff --git a/net/vmw_vsock/virtio_transport_common.c
b/net/vmw_vsock/virtio_transport_common.c
> index 6f1a8aff65c5..095221f94786 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/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:
> -- 
> 2.20.1

On Mon, Jul 29, 2019 at 10:04:29AM -0400, Michael S. Tsirkin
wrote:
> On Wed, Jul 17, 2019 at 01:30:26PM +0200, Stefano Garzarella wrote:
> > 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.
> > 
> > Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> > Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
> 
> This is good enough for net-next, but for net I think we
> should figure out how to address the issue completely.
> Can we make the accounting precise? What happens to
> performance if we do?
> 
In order to do more precise accounting maybe we can use the buffer size,
instead of payload size when we update the credit available.
In this way, the credit available for each socket will reflect the memory
actually used.

I should check better, because I'm not sure what happen if the peer sees
1KB of space available, then it sends 1KB of payload (using a 4KB
buffer).

The other option is to copy each packet in a new buffer like I did in
the v2 [2], but this forces us to make a copy for each packet that does
not fill the entire buffer, perhaps too expensive.

[2] https://patchwork.kernel.org/patch/10938741/


Thanks,
Stefano

On Mon, Jul 29, 2019 at 05:36:56PM +0200, Stefano Garzarella
wrote:
> On Mon, Jul 29, 2019 at 10:04:29AM -0400, Michael S. Tsirkin wrote:
> > On Wed, Jul 17, 2019 at 01:30:26PM +0200, Stefano Garzarella wrote:
> > > 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.
> > > 
> > > Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> > > Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
> > 
> > This is good enough for net-next, but for net I think we
> > should figure out how to address the issue completely.
> > Can we make the accounting precise? What happens to
> > performance if we do?
> > 
> 
> In order to do more precise accounting maybe we can use the buffer size,
> instead of payload size when we update the credit available.
> In this way, the credit available for each socket will reflect the memory
> actually used.
> 
> I should check better, because I'm not sure what happen if the peer
sees
> 1KB of space available, then it sends 1KB of payload (using a 4KB
> buffer).
> 
> The other option is to copy each packet in a new buffer like I did in
> the v2 [2], but this forces us to make a copy for each packet that does
> not fill the entire buffer, perhaps too expensive.
> 
> [2] https://patchwork.kernel.org/patch/10938741/
> 
> 
> Thanks,
> Stefano
Interesting. You are right, and at some level the protocol forces copies.

We could try to detect that the actual memory is getting close to
admin limits and force copies on queued packets after the fact.
Is that practical?

And yes we can extend the credit accounting to include buffer size.
That's a protocol change but maybe it makes sense.

-- 
MST

On Mon, Jul 29, 2019 at 05:36:56PM +0200, Stefano Garzarella
wrote:
> On Mon, Jul 29, 2019 at 10:04:29AM -0400, Michael S. Tsirkin wrote:
> > On Wed, Jul 17, 2019 at 01:30:26PM +0200, Stefano Garzarella wrote:
> > > 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.
> > > 
> > > Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> > > Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
> > 
> > This is good enough for net-next, but for net I think we
> > should figure out how to address the issue completely.
> > Can we make the accounting precise? What happens to
> > performance if we do?
> > 
> 
> In order to do more precise accounting maybe we can use the buffer size,
> instead of payload size when we update the credit available.
> In this way, the credit available for each socket will reflect the memory
> actually used.
> 
> I should check better, because I'm not sure what happen if the peer
sees
> 1KB of space available, then it sends 1KB of payload (using a 4KB
> buffer).
> The other option is to copy each packet in a new buffer like I did in
> the v2 [2], but this forces us to make a copy for each packet that does
> not fill the entire buffer, perhaps too expensive.
> 
> [2] https://patchwork.kernel.org/patch/10938741/
> 
So one thing we can easily do is to under-report the
available credit. E.g. if we copy up to 256bytes,
then report just 256bytes for every buffer in the queue.

> 
> Thanks,
> Stefano