Commit 59104f06 authored Sep 20, 2010 by Eric Dumazet Committed by David S. Miller Sep 24, 2010

ip: take care of last fragment in ip_append_data



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>

parent a02cec21

net/ipv4/ip_output.c

+6 −3

Original line number	Diff line number	Diff line
		@@ -926,16 +926,19 @@ int ip_append_data(struct sock *sk,
		!(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,