Question

<sk_buff 有两个地方可以储存下一个分散数据:

skb_shinfo(head)->frag_list 
skb_shinfo(head)->frags[]

这两种处理分裂问题的方法有何差别?

Answer 1

这两种情况都用于不同的案件。

<><> >

When your device supports scatter-gather I/O, and you want it to do the combining of data, etc., you can populate the frags[] structure starting with the second fragment till the nth fragment. The first fragment is always specified by the data and tail pointers. The rest of the fragments are filled in the frags[] structure. If you don t use scatter gather, this variable is empty.

<><>光线>

这是IP碎片清单。将在<代码>ip_push_pending_frames上填满。

页: 1 符合这一安排。

sk_buff0->next = sk_buff1
sk_buff1->next = sk_buff2
...
sk_buffn-1->next = sk_buffn

在<代码>ip_push_pending_frames之后,将

sk_buff0->frag_list = sk_buff1
sk_buff1->next = sk_buff2
...
sk_buffn-1->next = sk_buffn

简单化

frags[] are for scatter-gather I/O buffers
frag_list is for IP fragments

Answer 2

<>skb_shinfo(head)->frags[]

If the NIC supports SG I/O, __ip_append_data will copy user space data to skb_shinfo(head)->frags. The NIC driver (e.g., ixgbe_add_rx_frag) can also use these frags[] to carry the received network traffic; please note that every content in frags[] is a part of a complete packet. A complete packet consists of all frags[] + (skb->data ~ skb->tail).

skb_shinfo(head)->frag_list

This member is not used by IP fragmentation directly. In __ip_make_skb(), the frag_list is used to collect all skbs from sk->sk_write_queue; some NIC drivers also use this frag_list for carrying a packet to the upper network stack. Every content/skb in frag_list is also not a complete packet; tcp_v4_send_ack -> ip_send_unicast_reply -> ip_push_pending_frames -> ip_finish_skb -> __ip_make_skb;

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