Subject: Re: [PATCH net-next] tun/macvtap: limit the packets queued through rcvbuf

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 14 Jan 2014 10:25:16 +0200

On Tue, Jan 14, 2014 at 02:53:07PM +0800, Jason Wang wrote:
> We used to limit the number of packets queued through tx_queue_length. This
> has several issues:
> 
> - tx_queue_length is the control of qdisc queue length, simply reusing it
>   to control the packets queued by device may cause confusion.
> - After commit 6acf54f1cf0a6747bac9fea26f34cfc5a9029523 ("macvtap: Add
>   support of packet capture on macvtap device."), an unexpected qdisc
>   caused by non-zero tx_queue_length will lead qdisc lock contention for
>   multiqueue deivce.
> - What we really want is to limit the total amount of memory occupied not
>   the number of packets.
> 
> So this patch tries to solve the above issues by using socket rcvbuf to
> limit the packets could be queued for tun/macvtap. This was done by using
> sock_queue_rcv_skb() instead of a direct call to skb_queue_tail(). Also two
> new ioctl() were introduced for userspace to change the rcvbuf like what we
> have done for sndbuf.
> 
> With this fix, we can safely change the tx_queue_len of macvtap to
> zero. This will make multiqueue works without extra lock contention.
> 
> Cc: Vlad Yasevich <[email-masked]>
> Cc: Michael S. Tsirkin <[email-masked]>
> Cc: John Fastabend <[email-masked]>
> Cc: Stephen Hemminger <[email-masked]>
> Cc: Herbert Xu <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>

No, I don't think we can change userspace-visible behaviour like that.

This will break any existing user that tries to control
queue length through sysfs,netlink or device ioctl.

Take a look at my patch in msg ID [email-masked]
which gives one way to set tx_queue_len to zero without
breaking userspace.

> ---
>  drivers/net/macvtap.c       | 31 ++++++++++++++++++++---------
>  drivers/net/tun.c           | 48 +++++++++++++++++++++++++++++++++------------
>  include/uapi/linux/if_tun.h |  3 +++
>  3 files changed, 60 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index a2c3a89..c429c56 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -292,9 +292,6 @@ static rx_handler_result_t macvtap_handle_frame(struct sk_buff **pskb)
>  	if (!q)
>  		return RX_HANDLER_PASS;
>  
> -	if (skb_queue_len(&q->sk.sk_receive_queue) >= dev->tx_queue_len)
> -		goto drop;
> -
>  	skb_push(skb, ETH_HLEN);
>  
>  	/* Apply the forward feature mask so that we perform segmentation
> @@ -310,8 +307,10 @@ static rx_handler_result_t macvtap_handle_frame(struct sk_buff **pskb)
>  			goto drop;
>  
>  		if (!segs) {
> -			skb_queue_tail(&q->sk.sk_receive_queue, skb);
> -			goto wake_up;
> +			if (sock_queue_rcv_skb(&q->sk, skb))
> +				goto drop;
> +			else
> +				goto wake_up;
>  		}
>  
>  		kfree_skb(skb);
> @@ -319,11 +318,17 @@ static rx_handler_result_t macvtap_handle_frame(struct sk_buff **pskb)
>  			struct sk_buff *nskb = segs->next;
>  
>  			segs->next = NULL;
> -			skb_queue_tail(&q->sk.sk_receive_queue, segs);
> +			if (sock_queue_rcv_skb(&q->sk, segs)) {
> +				skb = segs;
> +				skb->next = nskb;
> +				goto drop;
> +			}
> +
>  			segs = nskb;
>  		}
>  	} else {
> -		skb_queue_tail(&q->sk.sk_receive_queue, skb);
> +		if (sock_queue_rcv_skb(&q->sk, skb))
> +			goto drop;
>  	}
>  
>  wake_up:
> @@ -333,7 +338,7 @@ wake_up:
>  drop:
>  	/* Count errors/drops only here, thus don't care about args. */
>  	macvlan_count_rx(vlan, 0, 0, 0);
> -	kfree_skb(skb);
> +	kfree_skb_list(skb);
>  	return RX_HANDLER_CONSUMED;
>  }
>  
> @@ -414,7 +419,7 @@ static void macvtap_dellink(struct net_device *dev,
>  static void macvtap_setup(struct net_device *dev)
>  {
>  	macvlan_common_setup(dev);
> -	dev->tx_queue_len = TUN_READQ_SIZE;
> +	dev->tx_queue_len = 0;
>  }
>  
>  static struct rtnl_link_ops macvtap_link_ops __read_mostly = {
> @@ -469,6 +474,7 @@ static int macvtap_open(struct inode *inode, struct file *file)
>  	sock_init_data(&q->sock, &q->sk);
>  	q->sk.sk_write_space = macvtap_sock_write_space;
>  	q->sk.sk_destruct = macvtap_sock_destruct;
> +	q->sk.sk_rcvbuf = TUN_RCVBUF;
>  	q->flags = IFF_VNET_HDR | IFF_NO_PI | IFF_TAP;
>  	q->vnet_hdr_sz = sizeof(struct virtio_net_hdr);
>  
> @@ -1040,6 +1046,13 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
>  		q->sk.sk_sndbuf = u;
>  		return 0;
>  
> +	case TUNSETRCVBUF:
> +		if (get_user(u, up))
> +			return -EFAULT;
> +
> +		q->sk.sk_rcvbuf = u;
> +		return 0;
> +
>  	case TUNGETVNETHDRSZ:
>  		s = q->vnet_hdr_sz;
>  		if (put_user(s, sp))
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 09f6662..7a08fa3 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -177,6 +177,7 @@ struct tun_struct {
>  
>  	int			vnet_hdr_sz;
>  	int			sndbuf;
> +	int			rcvbuf;
>  	struct tap_filter	txflt;
>  	struct sock_fprog	fprog;
>  	/* protected by rtnl lock */
> @@ -771,17 +772,6 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	if (!check_filter(&tun->txflt, skb))
>  		goto drop;
>  
> -	if (tfile->socket.sk->sk_filter &&
> -	    sk_filter(tfile->socket.sk, skb))
> -		goto drop;
> -
> -	/* Limit the number of packets queued by dividing txq length with the
> -	 * number of queues.
> -	 */
> -	if (skb_queue_len(&tfile->socket.sk->sk_receive_queue)
> -			  >= dev->tx_queue_len / tun->numqueues)
> -		goto drop;
> -
>  	if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))
>  		goto drop;
>  
> @@ -798,7 +788,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	nf_reset(skb);
>  
>  	/* Enqueue packet */
> -	skb_queue_tail(&tfile->socket.sk->sk_receive_queue, skb);
> +	if (sock_queue_rcv_skb(tfile->socket.sk, skb))
> +		goto drop;
>  
>  	/* Notify and wake up reader process */
>  	if (tfile->flags & TUN_FASYNC)
> @@ -1668,6 +1659,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  
>  		tun->filter_attached = false;
>  		tun->sndbuf = tfile->socket.sk->sk_sndbuf;
> +		tun->rcvbuf = tfile->socket.sk->sk_rcvbuf;
>  
>  		spin_lock_init(&tun->lock);
>  
> @@ -1837,6 +1829,17 @@ static void tun_set_sndbuf(struct tun_struct *tun)
>  	}
>  }
>  
> +static void tun_set_rcvbuf(struct tun_struct *tun)
> +{
> +	struct tun_file *tfile;
> +	int i;
> +
> +	for (i = 0; i < tun->numqueues; i++) {
> +		tfile = rtnl_dereference(tun->tfiles[i]);
> +		tfile->socket.sk->sk_sndbuf = tun->sndbuf;
> +	}
> +}
> +
>  static int tun_set_queue(struct file *file, struct ifreq *ifr)
>  {
>  	struct tun_file *tfile = file->private_data;
> @@ -1878,7 +1881,7 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  	struct ifreq ifr;
>  	kuid_t owner;
>  	kgid_t group;
> -	int sndbuf;
> +	int sndbuf, rcvbuf;
>  	int vnet_hdr_sz;
>  	unsigned int ifindex;
>  	int ret;
> @@ -2061,6 +2064,22 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  		tun_set_sndbuf(tun);
>  		break;
>  
> +	case TUNGETRCVBUF:
> +		rcvbuf = tfile->socket.sk->sk_rcvbuf;
> +		if (copy_to_user(argp, &rcvbuf, sizeof(rcvbuf)))
> +			ret = -EFAULT;
> +		break;
> +
> +	case TUNSETRCVBUF:
> +		if (copy_from_user(&rcvbuf, argp, sizeof(rcvbuf))) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		tun->rcvbuf = rcvbuf;
> +		tun_set_rcvbuf(tun);
> +		break;
> +
>  	case TUNGETVNETHDRSZ:
>  		vnet_hdr_sz = tun->vnet_hdr_sz;
>  		if (copy_to_user(argp, &vnet_hdr_sz, sizeof(vnet_hdr_sz)))
> @@ -2139,6 +2158,8 @@ static long tun_chr_compat_ioctl(struct file *file,
>  	case TUNSETTXFILTER:
>  	case TUNGETSNDBUF:
>  	case TUNSETSNDBUF:
> +	case TUNGETRCVBUF:
> +	case TUNSETRCVBUF:
>  	case SIOCGIFHWADDR:
>  	case SIOCSIFHWADDR:
>  		arg = (unsigned long)compat_ptr(arg);
> @@ -2204,6 +2225,7 @@ static int tun_chr_open(struct inode *inode, struct file * file)
>  
>  	tfile->sk.sk_write_space = tun_sock_write_space;
>  	tfile->sk.sk_sndbuf = INT_MAX;
> +	tfile->sk.sk_rcvbuf = TUN_RCVBUF;
>  
>  	file->private_data = tfile;
>  	set_bit(SOCK_EXTERNALLY_ALLOCATED, &tfile->socket.flags);
> diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
> index e9502dd..8e04657 100644
> --- a/include/uapi/linux/if_tun.h
> +++ b/include/uapi/linux/if_tun.h
> @@ -22,6 +22,7 @@
>  
>  /* Read queue size */
>  #define TUN_READQ_SIZE	500
> +#define TUN_RCVBUF	(512 * PAGE_SIZE)
>  
>  /* TUN device flags */
>  #define TUN_TUN_DEV 	0x0001	

That's about 16x less than we were able to queue previously
by default.
How can you be sure this won't break any applications?

> @@ -58,6 +59,8 @@
>  #define TUNSETQUEUE  _IOW('T', 217, int)
>  #define TUNSETIFINDEX	_IOW('T', 218, unsigned int)
>  #define TUNGETFILTER _IOR('T', 219, struct sock_fprog)
> +#define TUNGETRCVBUF   _IOR('T', 220, int)
> +#define TUNSETRCVBUF   _IOW('T', 221, int)
>  
>  /* TUNSETIFF ifr flags */
>  #define IFF_TUN		0x0001
> -- 
> 1.8.3.2