net: pskb_expand_head() optimization

pskb_expand_head() blindly takes references on fragments before calling
skb_release_data(), potentially releasing these references.

We can add a fast path, avoiding these atomic operations, if we own the
last reference on skb->head.

Based on a previous patch from David

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 20 insertions, 5 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c030cf894f57..2d1bc761fe4b 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -779,6 +779,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
         u8 *data;
         int size = nhead + (skb_end_pointer(skb) - skb->head) + ntail;
         long off;
+        bool fastpath;
 
         BUG_ON(nhead < 0);
 
@@ -800,14 +801,28 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
                skb_shinfo(skb),
                offsetof(struct skb_shared_info, frags[skb_shinfo(skb)->nr_frags]));
 
-        for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
-                get_page(skb_shinfo(skb)->frags[i].page);
+        /* Check if we can avoid taking references on fragments if we own
+         * the last reference on skb->head. (see skb_release_data())
+         */
+        if (!skb->cloned)
+                fastpath = true;
+        else {
+                int delta = skb->nohdr ? (1 << SKB_DATAREF_SHIFT) + 1 : 1;
 
-        if (skb_has_frag_list(skb))
-                skb_clone_fraglist(skb);
+                fastpath = atomic_read(&skb_shinfo(skb)->dataref) == delta;
+        }
 
-        skb_release_data(skb);
+        if (fastpath) {
+                kfree(skb->head);
+        } else {
+                for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
+                        get_page(skb_shinfo(skb)->frags[i].page);
 
+                if (skb_has_frag_list(skb))
+                        skb_clone_fraglist(skb);
+
+                skb_release_data(skb);
+        }
         off = (data + nhead) - skb->head;
 
         skb->head     = data;