netdev - [PATCH] tcp: allow effective reduction of TCP's rcv-buffer via setsockopt

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Date:	Thu, 19 Aug 2010 18:33:05 +0200
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	netdev@...r.kernel.org
Cc:	Hagen Paul Pfeifer <hagen@...u.net>,
	"David S. Miller" <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: [PATCH] tcp: allow effective reduction of TCP's rcv-buffer via setsockopt

Via setsockopt it is possible to reduce the socket RX buffer
(SO_RCVBUF). TCP method to select the initial window and window scaling
option in tcp_select_initial_window() currently misbehaves and do not
consider a reduced RX socket buffer via setsockopt.

Even though the server's RX buffer is reduced via setsockopt() to 256
byte (Initial Window 384 byte => 256 * 2 - (256 * 2 / 4)) the window
scale option is still 7:

192.168.1.38.40676 > 78.47.222.210.5001: Flags [S], seq 2577214362, win 5840, options [mss 1460,sackOK,TS val 338417 ecr 0,nop,wscale 0], length 0
78.47.222.210.5001 > 192.168.1.38.40676: Flags [S.], seq 1570631029, ack 2577214363, win 384, options [mss 1452,sackOK,TS val 2435248895 ecr 338417,nop,wscale 7], length 0
192.168.1.38.40676 > 78.47.222.210.5001: Flags [.], ack 1, win 5840, options [nop,nop,TS val 338421 ecr 2435248895], length 0

Within tcp_select_initial_window() the original space argument - a
representation of the rx buffer size - is expanded during
tcp_select_initial_window(). Only sysctl_tcp_rmem[2], sysctl_rmem_max
and window_clamp are considered to calculate the initial window.

This patch adjust the window_clamp argument if the user explicitly
reduce the receive buffer.

Signed-off-by: Hagen Paul Pfeifer <hagen@...u.net>
Cc: David S. Miller <davem@...emloft.net>
Cc: Patrick McHardy <kaber@...sh.net>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
---
 net/ipv4/tcp_output.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index de3bd84..01b94b8 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2429,6 +2429,12 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 		__u8 rcv_wscale;
 		/* Set this up on the first call only */
 		req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
+
+		/* limit the window selection if the user enforce a smaller rx buffer */
+		if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
+		    (req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
+			req->window_clamp = tcp_full_space(sk);
+
 		/* tcp_full_space because it is guaranteed to be the first packet */
 		tcp_select_initial_window(tcp_full_space(sk),
 			mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
@@ -2555,6 +2561,11 @@ static void tcp_connect_init(struct sock *sk)
 
 	tcp_initialize_rcv_mss(sk);
 
+	/* limit the window selection if the user enforce a smaller rx buffer */
+	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
+	    (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
+		tp->window_clamp = tcp_full_space(sk);
+
 	tcp_select_initial_window(tcp_full_space(sk),
 				  tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
 				  &tp->rcv_wnd,
-- 
1.7.2.1.95.g3d045.dirty

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