Subject: Re: tun mq failure

From: Jason Wang <[email-masked]>
Date: Wed, 23 Jan 2013 20:10:35 +0800

On 01/23/2013 07:41 PM, Michael S. Tsirkin wrote:
> On Wed, Jan 23, 2013 at 12:06:40PM +0100, Hannes Frederic Sowa wrote:
>> On Wed, Jan 23, 2013 at 12:05:16PM +0200, Michael S. Tsirkin wrote:
>>> This is when trying to start a VPN using some old openvpn binary so MQ
>>> is not set.
>>>
>>> So
>>> 1. I think we should limit allocation of MQ to when MQ flag is set in SETIFF.
>>> 2. order 7 allocation is 2^^7 pages - about half a megabyte of contigious
>>>    memory. This is quite likely to fail.
>>>    Let's start with a small limit on number of queues, like 8?
>>>    Then we know it will succeed.
>>>    Longer term we might want to solve it differently.
>> This has been come up before:
>> http://thread.gmane.org/gmane.linux.network/255647/focus=255902
>>
>> I think a solution to this problem is still outstanding.
> Right. What (at least I) missed is that it's the
> queue array allocation that fails here.
> So I think something like the following will sort the first issue
> (compiled only):
>
> For the second, for 3.8 maybe the prudent thing to do is
> to set MAX_TAP_QUEUES to a small value, like 8, to avoid
> userspace relying on a large number of queues being available,
> and look at a better way to do this longer term, like
> using an array of pointers.

Sure, this is just the method I reply in that thread. Not sure 8 is the
best, but since it fit into one page, should be ok. Maybe we can use
flex array to avoid high order memory allocation in the longer term.
>
> --->
>
> tun: don't waste memory on unused queues
>
> If MQ flag is off, we never attach more than 1 queue.
> So let's not allocate memory for the unused ones.
>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
>
> ---
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index af372d0..813d303 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1577,6 +1577,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  	else {
>  		char *name;
>  		unsigned long flags = 0;
> +		unsigned int max_tap_queues;
>  
>  		if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
>  			return -EPERM;
> @@ -1599,9 +1600,13 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  		if (*ifr->ifr_name)
>  			name = ifr->ifr_name;
>  
> +		if (ifr->ifr_flags & IFF_MULTI_QUEUE)
> +			max_tap_queues = MAX_TAP_QUEUES;
> +		else
> +			max_tap_queues = 1;
>  		dev = alloc_netdev_mqs(sizeof(struct tun_struct), name,
>  				       tun_setup,
> -				       MAX_TAP_QUEUES, MAX_TAP_QUEUES);
> +				       max_tap_queues, max_tap_queues);
>  		if (!dev)
>  			return -ENOMEM;
>