[ 38/47] skb: avoid unnecessary reallocations in __skb_cow

Greg KH
2012-06-07T04:25:33Z

From: Felix Fietkau <nbd@openwrt.org>

[ 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@openwrt.org>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/skbuff.h | 2 --
1 file changed, 2 deletions(-)

--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1633,8 +1633,6 @@ static inline int __skb_cow(struct sk_bu
{
int delta = 0;

- if (headroom < NET_SKB_PAD)
- headroom = NET_SKB_PAD;
if (headroom > skb_headroom(skb))
delta = headroom - skb_headroom(skb);