From: Alexey Kodanev <alexey.koda...@oracle.com>
Date: Thu, 19 Jan 2017 16:36:39 +0300
> Found that if we run LTP netstress test with large MSS (65K),
> the first attempt from server to send data comparable to this
> MSS on fastopen connection will be delayed by the probe timer.
>
> Here is an example:
>
> < S seq 0:0 win 43690 options [mss 65495 wscale 7 tfo cookie] length 32
> > S. seq 0:0 ack 1 win 43690 options [mss 65495 wscale 7] length 0
> < . ack 1 win 342 length 0
>
> Inside tcp_sendmsg(), tcp_send_mss() returns max MSS in 'mss_now',
> as well as in 'size_goal'. This results the segment not queued for
> transmition until all the data copied from user buffer. Then, inside
> __tcp_push_pending_frames(), it breaks on send window test and
> continues with the check probe timer.
>
> Fragmentation occurs in tcp_write_wakeup()...
>
> +0.2 > P. seq 1:43777 ack 1 win 342 length 43776
> < . ack 43777, win 1365 length 0
> > P. seq 43777:65001 ack 1 win 342 options [...] length 21224
> ...
>
> This also contradicts with the fact that we should bound to the half
> of the window if it is large.
>
> Fix this flaw by correctly initializing max_window. Before that, it
> could have large values that affect further calculations of 'size_goal'.
>
> Fixes: 168a8f58059a ("tcp: TCP Fast Open Server - main code path")
> Signed-off-by: Alexey Kodanev <alexey.koda...@oracle.com>
Applied and queued up for -stable, thanks.
