Commit 88078d98 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

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: default avatarEric Dumazet <>
Signed-off-by: default avatarDavid S. Miller <>
parent 292eba02
......@@ -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)
......@@ -1839,6 +1839,20 @@ done:
/* 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);
* __pskb_pull_tail - advance tail of skb header
* @skb: buffer to reallocate
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