Additional header to IPv4 packet can be segmented with GSO?

Question

I'm getting trouble with packet segmentation. I've already read from many sources about GSO, which is a generalized way for segmenting a packet with size greater than the Ethernet MTU (1500 B). However, I have not found an answer for doubts that I have in mind.

If we add a new set of bytes (ex. a new header by the name 'NH') between L2 and L3 layer, the kernel must be able to pass through NH and adjust sk_buff pointer to the beginning of the L3 to offload the packet according to the 'policy' of the L3 protocol type (ex. IPv4 fragmentation). My thoughts were to modify skb_network_protocol() function. This function, if I'm not wrong, enables skb_mac_gso_segment() to properly call GSO function for different types of L3 protocol. However, I'm not being able to segment my packets properly.

I have a kernel module that forwards packets through the network (OVS, Open vSwitch). On the tests which I've been running (h1 --ping-- h2), the host generates large ICMP packets and then sends packets which are less or equal than MTU size. Those packets are received by the first switch which attaches the new header NH, so if a packet had 1500B, it becomes 1500B + NH length. Here is the problem, the switch has already received a fragmented packet from the host, and the switch adds more bytes in the packet (kind of VLAN does).

Therefore, at first, I tried to ping large packets, but it didn't work. In OVS, before calling dev_queue_xmit(), a packet can be segmented by calling skb_gso_segment(). However, the packet needs to go through a condition checked by netif_needs_gso(). But I'm not sure if I have to use skb_gso_segment() to properly segment the packet.

I also noticed that, for the needs_gso_segment() function be true, skb_shinfo(skb)->gso_size have to be true. However, gso_size has always zero value for all the received packets. So, I made a test by attributing a random value to gso_size (ex. 1448B). Now, on my tests, I was able to ping from h1 to h2, but the first 2 packets were lost. On another test, TCP had a extremely poor performance. And since then, I've been getting a kernel warning: "[ 5212.694418] [c1642e50] ? skb_warn_bad_offload+0xd0/0xd8 "

For small packets (< MTU) I got no trouble and ping works fine. TCP works fine, but for small window size.

Someone has any idea for what's happening? Should I always use GSO when I get large packets? Is it possible to fragment a fragmented IPv4 packets?

As the new header lies between L2 and L3, I guess the enlargement of a IPv4 packet due to the additional header, is similar to what happens with VLAN. How VLAN can handle the segmentation problem?

Thanks in advance,