[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>

2 files changed, 7 insertions, 13 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);
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 882436da9a3a..be354155b2f9 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -756,13 +756,9 @@ static inline int select_size(struct sock *sk, struct tcp_sock *tp)
 {
         int tmp = tp->mss_cache_std;
 
-        if (sk->sk_route_caps & NETIF_F_SG) {
-                int pgbreak = SKB_MAX_HEAD(MAX_TCP_HEADER);
+        if (sk->sk_route_caps & NETIF_F_SG)
+                tmp = 0;
 
-                if (tmp >= pgbreak &&
-                    tmp <= pgbreak + (MAX_SKB_FRAGS - 1) * PAGE_SIZE)
-                        tmp = pgbreak;
-        }
         return tmp;
 }
 
@@ -872,11 +868,6 @@ new_segment:
                                         tcp_mark_push(tp, skb);
                                         goto new_segment;
                                 } else if (page) {
-                                        /* If page is cached, align
-                                         * offset to L1 cache boundary
-                                         */
-                                        off = (off + L1_CACHE_BYTES - 1) &
-                                              ~(L1_CACHE_BYTES - 1);
                                         if (off == PAGE_SIZE) {
                                                 put_page(page);
                                                 TCP_PAGE(sk) = page = NULL;