diff options
Diffstat (limited to 'include/linux/skbuff.h')
| -rw-r--r-- | include/linux/skbuff.h | 29 |
1 files changed, 25 insertions, 4 deletions
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 613b9513f8b..c4619a428d9 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -941,6 +941,25 @@ static inline void skb_reserve(struct sk_buff *skb, int len) #define NET_IP_ALIGN 2 #endif +/* + * The networking layer reserves some headroom in skb data (via + * dev_alloc_skb). This is used to avoid having to reallocate skb data when + * the header has to grow. In the default case, if the header has to grow + * 16 bytes or less we avoid the reallocation. + * + * Unfortunately this headroom changes the DMA alignment of the resulting + * network packet. As for NET_IP_ALIGN, this unaligned DMA is expensive + * on some architectures. An architecture can override this value, + * perhaps setting it to a cacheline in size (since that will maintain + * cacheline alignment of the DMA). It must be a power of 2. + * + * Various parts of the networking layer expect at least 16 bytes of + * headroom, you should not reduce this. + */ +#ifndef NET_SKB_PAD +#define NET_SKB_PAD 16 +#endif + extern int ___pskb_trim(struct sk_buff *skb, unsigned int len, int realloc); static inline void __skb_trim(struct sk_buff *skb, unsigned int len) @@ -1030,9 +1049,9 @@ static inline void __skb_queue_purge(struct sk_buff_head *list) static inline struct sk_buff *__dev_alloc_skb(unsigned int length, gfp_t gfp_mask) { - struct sk_buff *skb = alloc_skb(length + 16, gfp_mask); + struct sk_buff *skb = alloc_skb(length + NET_SKB_PAD, gfp_mask); if (likely(skb)) - skb_reserve(skb, 16); + skb_reserve(skb, NET_SKB_PAD); return skb; } #else @@ -1070,13 +1089,15 @@ static inline struct sk_buff *dev_alloc_skb(unsigned int length) */ static inline int skb_cow(struct sk_buff *skb, unsigned int headroom) { - int delta = (headroom > 16 ? headroom : 16) - skb_headroom(skb); + int delta = (headroom > NET_SKB_PAD ? headroom : NET_SKB_PAD) - + skb_headroom(skb); if (delta < 0) delta = 0; if (delta || skb_cloned(skb)) - return pskb_expand_head(skb, (delta + 15) & ~15, 0, GFP_ATOMIC); + return pskb_expand_head(skb, (delta + (NET_SKB_PAD-1)) & + ~(NET_SKB_PAD-1), 0, GFP_ATOMIC); return 0; } |