实现功能：
使用自己已经分配的内存让skb->data指向，而不是使用alloc_malloc()。
部分代码如下：
/*
* build a new sk_buff
*/
//struct sk_buff *send_skb = kmem_cache_alloc_node(skbuff_head_cache, GFP_ATOMIC & ~__GFP_DMA, NUMA_NO_NODE);
struct sk_buff *send_skb = kmem_cache_alloc(skbuff_head_cache, GFP_ATOMIC & ~__GFP_DMA); if (!send_skb) {
//spin_unlock(&lock);
return NF_DROP;
} //printk("what2\n");
memset(send_skb, , offsetof(struct sk_buff, tail));
atomic_set(&send_skb->users, );
send_skb->cloned = ; send_skb->head = mmap_buf + ;
send_skb->data = mmap_buf + ;
第18行，mmap_buf是提前分配的内存。
在/var/log/messages中网卡驱动会输出错误信息：
ep :: 10g-host2 kernel: ixgbe ::00.0: eth2: Detected Tx Unit Hang
Sep :: 10g-host2 kernel: Tx Queue <>
Sep :: 10g-host2 kernel: TDH, TDT <>, <1ea>
Sep :: 10g-host2 kernel: next_to_use <1ea>
Sep :: 10g-host2 kernel: next_to_clean <>
Sep :: 10g-host2 kernel: ixgbe ::00.0: eth2: Detected Tx Unit Hang
Sep :: 10g-host2 kernel: Tx Queue <>
Sep :: 10g-host2 kernel: TDH, TDT <>, <1eb>
Sep :: 10g-host2 kernel: next_to_use <1eb>
Sep :: 10g-host2 kernel: next_to_clean <>
Sep :: 10g-host2 kernel: ixgbe ::00.0: eth2: Detected Tx Unit Hang
Sep :: 10g-host2 kernel: Tx Queue <>
Sep :: 10g-host2 kernel: TDH, TDT <>, <1ea>
Sep :: 10g-host2 kernel: next_to_use <1ea>
Sep :: 10g-host2 kernel: next_to_clean <>
Sep :: 10g-host2 kernel: ixgbe ::00.0: eth2: Detected Tx Unit Hang
Sep :: 10g-host2 kernel: Tx Queue <>
Sep :: 10g-host2 kernel: TDH, TDT <>, <1ea>
Sep :: 10g-host2 kernel: next_to_use <1ea>
Sep :: 10g-host2 kernel: next_to_clean <>
Sep :: 10g-host2 kernel: ixgbe ::00.0: eth2: Detected Tx Unit Hang
Sep :: 10g-host2 kernel: Tx Queue <>
Sep :: 10g-host2 kernel: TDH, TDT <>, <1ef>
Sep :: 10g-host2 kernel: next_to_use <1ef>
Sep :: 10g-host2 kernel: next_to_clean <>
Sep :: 10g-host2 kernel: ixgbe ::00.0: eth2: Detected Tx Unit Hang
Sep :: 10g-host2 kernel: Tx Queue <>
Sep :: 10g-host2 kernel: TDH, TDT <>, <1ec>
Sep :: 10g-host2 kernel: next_to_use <1ec>
Sep :: 10g-host2 kernel: next_to_clean <>
Sep :: 10g-host2 kernel: ixgbe ::00.0: eth2: Detected Tx Unit Hang
在排除各种原因后，定位为分配的mmap_buf存在问题。使用vmalloc()分配不正确，改为kmalloc()后正常。
《Linux内核设计与实现》第12.5节有解释，应该是：网卡设备要求分配的物理地址连续，而vmalloc()只是虚拟地址连续