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SubjectRe: [PATCH] ip : take care of last fragment in ip_append_data
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Tue, 21 Sep 2010 08:16:27 +0200

> [PATCH] 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>

This patch largely looks fine, but:

1) I want to find out where that "17" tailer_len comes from before
applying this, that doesn't make any sense.

2) Even with #1 addressed, this function is tricky so I want to review
this patch some more.


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