[PATCH] skb: avoid unnecessary reallocations in __skb_cow

[PATCH] skb: avoid unnecessary reallocations in __skb_cow

[Felix Fietkau]

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>
---
 include/linux/skbuff.h |    2 --
 1 files changed, 0 insertions(+), 2 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0e50171..b534a1b 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1896,8 +1896,6 @@ 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);
 
-- 
1.7.3.2

Re: [PATCH] skb: avoid unnecessary reallocations in __skb_cow

[Eric Dumazet]

On Tue, 2012-05-29 at 15:35 +0200, Felix Fietkau wrote:

> 
> Signed-off-by: Felix Fietkau <nbd@openwrt.org>
> ---
>  include/linux/skbuff.h |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 

 

Signed-off-by: Eric Dumazet <edumazet@google.com>

Thanks !

Re: [PATCH] skb: avoid unnecessary reallocations in __skb_cow

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Tue, 29 May 2012 15:43:00 +0200

> On Tue, 2012-05-29 at 15:35 +0200, Felix Fietkau wrote:
> 
>> 
>> Signed-off-by: Felix Fietkau <nbd@openwrt.org>
>> ---
>>  include/linux/skbuff.h |    2 --
>>  1 files changed, 0 insertions(+), 2 deletions(-)
>> 
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Applied.