结构体 skb_shared_info
skb_shared_info
skb_shared_info 的定义
/* This data is invariant across clones and lives at
* the end of the header data, ie. at skb->end.
*/
struct skb_shared_info {
atomic_t dataref;
unsigned short nr_frags;
unsigned short gso_size;
/* Warning: this field is not always filled in (UFO)! */
unsigned short gso_segs;
unsigned short gso_type;
__be32 ip6_frag_id;
struct sk_buff *frag_list;
skb_frag_t frags[MAX_SKB_FRAGS];
};
用于skb clone的字段：
dataref : This field contains the reference count for this skb. It is incremented each time the buffer is cloned.
用于TCP 分段的字段：
nr_frags : the number of fragments in this packet. This field is used by TCP segmentation.
frags : the array of page table entries. Each entry is actually a TCP segment.
用于IP 分片的字段：
frag_list : This field points to the list of fragments for this packet if it is fragmented.
skb_shared_info 内存中的位置
This structure is placed at the end of the attached data buffer and pointed to by the end field
in the socket buffer structure(struct sk_buff), which points to the end of the data portion of the packet.
430 #ifdef NET_SKBUFF_DATA_USES_OFFSET
431 static inline unsigned char *skb_end_pointer(const struct sk_buff *skb)
432 {
433 return skb->head + skb->end;
434 }
435 #else
436 static inline unsigned char *skb_end_pointer(const struct sk_buff *skb)
437 {
438 return skb->end;
439 }
440 #endif
441
442 /* Internal */
443 #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))
skb_shared_info 的作用
- IP分片
- TCP分段
- 记录SKB的clone次数
- TODO GSO