Re: [PATCH] vti6: Add pmtu handling to vti6_xmit.

[Mark McKinstry <Mark.McKinstry@alliedtelesis.co.nz>]

I've tested this patch in our scenario and I can confirm that it still 
fixes all of our issues.

On 22/03/16 23:53, Steffen Klassert wrote:
> On Tue, Mar 15, 2016 at 01:28:01PM +0100, Steffen Klassert wrote:
>> On Mon, Mar 14, 2016 at 09:52:05PM +0000, Mark McKinstry wrote:
>>> Your patch adds a dst_release() call to my suggested fix, but this is
>>> problematic because the kfree_skb() call at tx_error already takes care
>>> of releasing dst - via kfree_skb() > __kfree_skb() > skb_release_all() >
>>> skb_release_head_state() > skb_dst_drop()
>>>   > refdst_drop() > dst_release(). In our scenario your patch results in
>>> a negative refcount kernel warning being generated in dst_release() for
>>> every packet that is too big to go over the vti.
>> Hm. I've just noticed that my pmtu test does not trigger this
>> codepath, so I did not see the warning.
>>
>> Seems like we do the pmtu handling too late, it should happen before
>> we do skb_dst_set(). Also skb_scrub_packet() resets skb->ignore_df,
>> so checking ignore_df after skb_scrub_packet() does not make much sense.
>>
>> I'll send an updated version after some more testing.
>>
> I've added a testcase that triggers this codepath to my testing
> environment. The patch below works for me, could you please test
> if it fixes your problems?
>
> Subject: [PATCH] vti: Add pmtu handling to vti_xmit.
>
> We currently rely on the PMTU discovery of xfrm.
> However if a packet is locally sent, the PMTU mechanism
> of xfrm tries to do local socket notification what
> might not work for applications like ping that don't
> check for this. So add pmtu handling to vti_xmit to
> report MTU changes immediately.
>
> Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
> ---
>   net/ipv4/ip_vti.c | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
>
> diff --git a/net/ipv4/ip_vti.c b/net/ipv4/ip_vti.c
> index 5cf10b7..a917903 100644
> --- a/net/ipv4/ip_vti.c
> +++ b/net/ipv4/ip_vti.c
> @@ -156,6 +156,7 @@ static netdev_tx_t vti_xmit(struct sk_buff *skb, struct net_device *dev,
>   	struct dst_entry *dst = skb_dst(skb);
>   	struct net_device *tdev;	/* Device to other host */
>   	int err;
> +	int mtu;
>   
>   	if (!dst) {
>   		dev->stats.tx_carrier_errors++;
> @@ -192,6 +193,23 @@ static netdev_tx_t vti_xmit(struct sk_buff *skb, struct net_device *dev,
>   			tunnel->err_count = 0;
>   	}
>   
> +	mtu = dst_mtu(dst);
> +	if (skb->len > mtu) {
> +		skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL, skb, mtu);
> +		if (skb->protocol == htons(ETH_P_IP)) {
> +			icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
> +				  htonl(mtu));
> +		} else {
> +			if (mtu < IPV6_MIN_MTU)
> +				mtu = IPV6_MIN_MTU;
> +
> +			icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
> +		}
> +
> +		dst_release(dst);
> +		goto tx_error;
> +	}
> +
>   	skb_scrub_packet(skb, !net_eq(tunnel->net, dev_net(dev)));
>   	skb_dst_set(skb, dst);
>   	skb->dev = skb_dst(skb)->dev;