[ 73/82] skb: avoid unnecessary reallocations in __skb_cow

[Ben Hutchings]

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Felix Fietkau <nbd@xxxxxxxxxxx>

[ Upstream commit 617c8c11236716dcbda877e764b7bf37c6fd8063 ]

At the beginning of __skb_cow, headroom gets set to a minimum of
NET_SKB_PAD. This causes unnecessary reallocations if the buffer was not
cloned and the headroom is just below NET_SKB_PAD, but still more than the
amount requested by the caller.
This was showing up frequently in my tests on VLAN tx, where
vlan_insert_tag calls skb_cow_head(skb, VLAN_HLEN).

Locally generated packets should have enough headroom, and for forward
paths, we already have NET_SKB_PAD bytes of headroom, so we don't need to
add any extra space here.

Signed-off-by: Felix Fietkau <nbd@xxxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/linux/skbuff.h |    2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index e689b47..bdb4590 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1866,8 +1866,6 @@ static inline int __skb_cow(struct sk_buff *skb, unsigned int headroom,
 {
 	int delta = 0;
 
-	if (headroom < NET_SKB_PAD)
-		headroom = NET_SKB_PAD;
 	if (headroom > skb_headroom(skb))
 		delta = headroom - skb_headroom(skb);
 


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