Patch "tcp: initialize max window for a new fastopen socket" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

tcp: initialize max window for a new fastopen socket

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tcp-initialize-max-window-for-a-new-fastopen-socket.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Alexey Kodanev <[email protected]>
Date: Thu, 19 Jan 2017 16:36:39 +0300
Subject: tcp: initialize max window for a new fastopen socket

From: Alexey Kodanev <[email protected]>

[ Upstream commit 0dbd7ff3ac5017a46033a9d0a87a8267d69119d9 ]

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'.

---
 net/ipv4/tcp_fastopen.c |    1 +
 1 file changed, 1 insertion(+)

@@ -161,6 +161,7 @@ static struct sock *tcp_fastopen_create_
 	 * scaled. So correct it appropriately.
 	 */
 	tp->snd_wnd = ntohs(tcp_hdr(skb)->window);
+	tp->max_window = tp->snd_wnd;
 
 	/* Activate the retrans timer so that SYNACK can be retransmitted.
 	 * The request socket is not added to the ehash

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/tcp-initialize-max-window-for-a-new-fastopen-socket.patch