netdev - skb_split in tcp_retransmit

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Date:	Thu, 27 Jan 2011 17:20:57 +0200
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Pekka Savola (ipv6)" <pekkas@...core.fi>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: skb_split in tcp_retransmit_skb question

Hello,

Suppose we have the following scenario:

tcp_write_timer ->
 tcp_retransmit_skb

in tcp_retransmit_skb we have `if (skb->len > cur_mss)' evaluted to true, which leads
to tcp_fragment(sk, skb, cur_mss, cur_mss) call. tcp_fragment calls skb_split(skb, buff, len)
which, in turn, calls skb_split_no_header(skb, skb1, len, pos), where we have
`skb_shinfo(skb)->nr_frags++' while in `for (i = 0; i < nfrags; i++)' loop.

Now we fall back to: 
  tcp_retransmit_skb ->
   tcp_transmit_skb ->
    pskb_copy(skb, gfp_mask)

In pskb_copy we perform iteration on nr_frags: 

 729     if (skb_shinfo(skb)->nr_frags) {
 730         int i;
 731         for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
 732             skb_shinfo(n)->frags[i] = skb_shinfo(skb)->frags[i];
 733             get_page(skb_shinfo(n)->frags[i].page);
 734         }
 735         skb_shinfo(n)->nr_frags = i;
 736     }

The problem here is that nr_frags was increased in skb_split, yet new page was not allocated.
So, get_page(skb_shinfo(n)->frags[i].page) is actually get_page(NULL):

	mov (%rdx), %eax
	where %rdx is 0x00

Please correct me if I'm missing something.

	Sergey

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