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ip: take care of last fragment in ip_append_data

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While investigating a bit, I found ip_fragment() slow path was taken
because ip_append_data() provides following layout for a send(MTU +
N*(MTU - 20)) syscall :

- one skb with 1500 (mtu) bytes
- N fragments of 1480 (mtu-20) bytes (before adding IP header)
last fragment gets 17 bytes of trail data because of following bit:

	if (datalen == length + fraggap)
		alloclen += rt->dst.trailer_len;

Then esp4 adds 16 bytes of data (while trailer_len is 17... hmm...
another bug ?)

In ip_fragment(), we notice last fragment is too big (1496 + 20) > mtu,
so we take slow path, building another skb chain.

In order to avoid taking slow path, we should correct ip_append_data()
to make sure last fragment has real trail space, under mtu...

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2010-09-20 20:16:27 +00:00 committed by David S. Miller
parent a02cec2155
commit 59104f0624
1 changed files with 6 additions and 3 deletions
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9
net/ipv4/ip_output.c
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@ -926,16 +926,19 @@ alloc_new_skb:
!(rt->dst.dev->features&NETIF_F_SG))
alloclen = mtu;
else
alloclen = datalen + fragheaderlen;
alloclen = fraglen;
/* The last fragment gets additional space at tail.
* Note, with MSG_MORE we overallocate on fragments,
* because we have no idea what fragment will be
* the last.
*/
if (datalen == length + fraggap)
if (datalen == length + fraggap) {
alloclen += rt->dst.trailer_len;
/* make sure mtu is not reached */
if (datalen > mtu - fragheaderlen - rt->dst.trailer_len)
datalen -= ALIGN(rt->dst.trailer_len, 8);
}
if (transhdrlen) {
skb = sock_alloc_send_skb(sk,
alloclen + hh_len + 15,