net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends

After working on IP defragmentation lately, I found that some large
packets defeat CHECKSUM_COMPLETE optimization because of NIC adding
zero paddings on the last (small) fragment.

While removing the padding with pskb_trim_rcsum(), we set skb->ip_summed
to CHECKSUM_NONE, forcing a full csum validation, even if all prior
fragments had CHECKSUM_COMPLETE set.

We can instead compute the checksum of the part we are trimming,
usually smaller than the part we keep.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -3131,6 +3131,7 @@ static inline void *skb_push_rcsum(struct sk_buff *skb, unsigned int len)
 	return skb->data;
 }
 
+int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len);
 /**
  *	pskb_trim_rcsum - trim received skb and update checksum
  *	@skb: buffer to trim
@@ -3144,9 +3145,7 @@ 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);
+	return pskb_trim_rcsum_slow(skb, len);
 }
 
 static inline int __skb_trim_rcsum(struct sk_buff *skb, unsigned int len)

net/core/skbuff.c

@@ -1839,6 +1839,20 @@ done:
 }
 EXPORT_SYMBOL(___pskb_trim);
 
+/* Note : use pskb_trim_rcsum() instead of calling this directly
+ */
+int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
+{
+	if (skb->ip_summed == CHECKSUM_COMPLETE) {
+		int delta = skb->len - len;
+
+		skb->csum = csum_sub(skb->csum,
+				     skb_checksum(skb, len, delta, 0));
+	}
+	return __pskb_trim(skb, len);
+}
+EXPORT_SYMBOL(pskb_trim_rcsum_slow);
+
 /**
  *	__pskb_pull_tail - advance tail of skb header
  *	@skb: buffer to reallocate