Why does tcp collapse behavior depend on nr_frags?

[Ilya Lesokhin]

Hi,
I've notice that tcp_can_collapse() returns false if skb_shinfo(skb)->nr_frags 
!= 0.
Is there a reason why we want to base the collapse decision in retransmission 
on whether
the data is located in a frag or the linear part?

The relevant commit is
         tcp: collapse more than two on retransmission  
('4a17fc3add594fcc1c778e93a95b6ecf47f630e5')

Thanks,
Ilya