Re: [PATCH v3] net: Do not include padding in TCP GRO checksum

[David Miller <davem@davemloft.net>]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Sat, 16 Nov 2013 08:47:38 +0800

> Also, if the padding occurs on every single packet or a fairly
> large amount of packets then you should consider improving
> pskb_trim_rcsum to keep the CHECKSUM_COMPLETE value.

It should be as easy as this?  (this is actually just a 3 line change,
it looks like more because I'm moving pskb_trim_rcsum() later in the
file to where skb_checksum() is in scope).

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 215b5ea..bec1cc7 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2263,24 +2263,6 @@ static inline void skb_postpull_rcsum(struct sk_buff *skb,
 
 unsigned char *skb_pull_rcsum(struct sk_buff *skb, unsigned int len);
 
-/**
- *	pskb_trim_rcsum - trim received skb and update checksum
- *	@skb: buffer to trim
- *	@len: new length
- *
- *	This is exactly the same as pskb_trim except that it ensures the
- *	checksum of received packets are still valid after the operation.
- */
-
-static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
-{
-	if (likely(len >= skb->len))
-		return 0;
-	if (skb->ip_summed == CHECKSUM_COMPLETE)
-		skb->ip_summed = CHECKSUM_NONE;
-	return __pskb_trim(skb, len);
-}
-
 #define skb_queue_walk(queue, skb) \
 		for (skb = (queue)->next;					\
 		     skb != (struct sk_buff *)(queue);				\
@@ -2378,6 +2360,27 @@ __wsum __skb_checksum(const struct sk_buff *skb, int offset, int len,
 __wsum skb_checksum(const struct sk_buff *skb, int offset, int len,
 		    __wsum csum);
 
+/**
+ *	pskb_trim_rcsum - trim received skb and update checksum
+ *	@skb: buffer to trim
+ *	@len: new length
+ *
+ *	This is exactly the same as pskb_trim except that it ensures the
+ *	checksum of received packets are still valid after the operation.
+ */
+
+static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
+{
+	if (likely(len >= skb->len))
+		return 0;
+	if (skb->ip_summed == CHECKSUM_COMPLETE) {
+		__wsum adj = skb_checksum(skb, len, skb->len - len, 0);
+
+		skb->csum = csum_sub(skb->csum, adj);
+	}
+	return __pskb_trim(skb, len);
+}
+
 static inline void *skb_header_pointer(const struct sk_buff *skb, int offset,
 				       int len, void *buffer)
 {