net: skb_segment() provides list head and tail

Its unfortunate we have to walk again skb list to find the tail
after segmentation, even if data is probably hot in cpu caches.

skb_segment() can store the tail of the list into segs->prev,
and validate_xmit_skb_list() can immediately get the tail.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 20 insertions, 12 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 1a90530f83ff..7d5691cc1f47 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2724,22 +2724,25 @@ struct sk_buff *validate_xmit_skb_list(struct sk_buff *skb, struct net_device *d
 {
         struct sk_buff *next, *head = NULL, *tail;
 
-        while (skb) {
+        for (; skb != NULL; skb = next) {
                 next = skb->next;
                 skb->next = NULL;
+
+                /* in case skb wont be segmented, point to itself */
+                skb->prev = skb;
+
                 skb = validate_xmit_skb(skb, dev);
-                if (skb) {
-                        struct sk_buff *end = skb;
+                if (!skb)
+                        continue;
 
-                        while (end->next)
-                                end = end->next;
-                        if (!head)
-                                head = skb;
-                        else
-                                tail->next = skb;
-                        tail = end;
-                }
-                skb = next;
+                if (!head)
+                        head = skb;
+                else
+                        tail->next = skb;
+                /* If skb was segmented, skb->prev points to
+                 * the last segment. If not, it still contains skb.
+                 */
+                tail = skb->prev;
         }
         return head;
 }
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 9a423e2c5766..7b3df0d518ab 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3083,6 +3083,11 @@ perform_csum_check:
                 }
         } while ((offset += len) < head_skb->len);
 
+        /* Some callers want to get the end of the list.
+         * Put it in segs->prev to avoid walking the list.
+         * (see validate_xmit_skb_list() for example)
+         */
+        segs->prev = tail;
         return segs;
 
 err: