in linux port_mirror,why skb_clone can change nskb->data?

Question

`int mirror_func(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt, struct net_device *orig_dev) { struct sk_buff *nskb;

if (skb_shared(skb) && is_ports(dev))
{
    nskb = skb_clone(skb, GFP_ATOMIC);
    if (!nskb)
        goto out;
    nskb->dev = mirror;
    nskb->len += nskb->mac_len;
    nskb->data -= nskb->mac_len;
    dev_queue_xmit(nskb);
}

out: kfree_skb(skb); return NET_RX_SUCCESS; }`

Q: this program is from github, running it is ok,but I still don't understand somewhere. In mirror_func, use skb_clone, nskb and skb share the same memmory, why can change nskb->data pointer, does skb->data also change? skb's TX/RX is affected?

And why kfree_skb(skb), skb's TX/RX is affected?

I try to: printk("skb=%p nskb=%p skb->data=%p,nskb->data=%p\n", skb, nskb, skb->data, nskb->data);

after adding printk(),result is: skb=d6135889 nskb=e5502745 skb->data=4a4ed991,nskb->data=0b75433f

why skb != nskb? why skb->data != nskb->data? Does this mean that skb and nskb are different, so why kfree_skb(skb)?I think that skb should TX to L1 or RX to L3.