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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 14 Jan 2014 11:52:19 +0200

On Tue, Jan 14, 2014 at 04:45:24PM +0800, Jason Wang wrote:
> On 01/14/2014 04:25 PM, Michael S. Tsirkin wrote:
> > 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.
> 
> But it looks like a buggy API, since tx_queue_len should be for qdisc
> queue length instead of device itself.

Probably, but it's been like this since 2.6.x time.
Also, qdisc queue is unused for tun so it seemed kind of
reasonable to override tx_queue_len.

> If we really want to preserve the
> behaviour, how about using a new feature flag and change the behaviour
> only when the device is created (TUNSETIFF) with the new flag?

OK this addresses the issue partially, but there's also an issue
of permissions: tx_queue_len can only be changed if
capable(CAP_NET_ADMIN). OTOH in your patch a regular user
can change the amount of memory consumed per queue
by calling TUNSETRCVBUF.

> >
> > 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.
> 
> If I read the patch correctly, it will make no way for the user who
> really want to change the qdisc queue length for tun.

Why would this matter?  As far as I can see qdisc queue is currently unused.

> >
> >
> >> ---
> >>  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?
> >
> 
> Ok, we can change it back to 500 * 65535, but I'm not sure whether this
> or not this value (about 32M) is too big for a single socket.
> >> @@ -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
> > --
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