[TCP]: Simplify SKB data portion allocation with NETIF_F_SG.

The ideal and most optimal layout for an SKB when doing
scatter-gather is to put all the headers at skb->data, and
all the user data in the page array.

This makes SKB splitting and combining extremely simple,
especially before a packet goes onto the wire the first
time.

So, when sk_stream_alloc_pskb() is given a zero size, make
sure there is no skb_tailroom().  This is achieved by applying
SKB_DATA_ALIGN() to the header length used here.

Next, make select_size() in TCP output segmentation use a
length of zero when NETIF_F_SG is true on the outgoing
interface.

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 2 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index e593af5b1ecc..7b76f891ae2d 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1134,13 +1134,16 @@ static inline void sk_stream_moderate_sndbuf(struct sock *sk)
 static inline struct sk_buff *sk_stream_alloc_pskb(struct sock *sk,
                                                    int size, int mem, int gfp)
 {
-        struct sk_buff *skb = alloc_skb(size + sk->sk_prot->max_header, gfp);
+        struct sk_buff *skb;
+        int hdr_len;
 
+        hdr_len = SKB_DATA_ALIGN(sk->sk_prot->max_header);
+        skb = alloc_skb(size + hdr_len, gfp);
         if (skb) {
                 skb->truesize += mem;
                 if (sk->sk_forward_alloc >= (int)skb->truesize ||
                     sk_stream_mem_schedule(sk, skb->truesize, 0)) {
-                        skb_reserve(skb, sk->sk_prot->max_header);
+                        skb_reserve(skb, hdr_len);
                         return skb;
                 }
                 __kfree_skb(skb);