linux-kernel - [PATCH] Revert "net: limit tcp/udp rmem/wmem to SOCK_{RCV,SND}BUF

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Message-ID: <20150813212134.GA3070917@mail.thefacebook.com>
Date:	Thu, 13 Aug 2015 14:21:34 -0700
From:	Calvin Owens <calvinowens@...com>
To:	David Miller <davem@...emloft.net>
CC:	<kuznet@....inr.ac.ru>, <jmorris@...ei.org>,
	<yoshfuji@...ux-ipv6.org>, <kaber@...sh.net>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<kernel-team@...com>, <sorin@...urnze.ro>
Subject: [PATCH] Revert "net: limit tcp/udp rmem/wmem to
 SOCK_{RCV,SND}BUF_MIN"

Commit 8133534c760d4083 ("net: limit tcp/udp rmem/wmem to
SOCK_{RCV,SND}BUF_MIN") modified four sysctls to enforce that the values
written to them are not less than SOCK_MIN_{RCV,SND}BUF.

That change causes 4096 to no longer be accepted as a valid value for
'min' in tcp_wmem and udp_wmem_min. 4096 has been the default for both
of those sysctls for a long time, and unfortunately seems to be an
extremely popular setting. This change breaks a large number of sysctl
configurations at Facebook.

That commit referred to b1cb59cf2efe7971 ("net: sysctl_net_core: check
SNDBUF and RCVBUF for min length"), which choose to use the SOCK_MIN
constants as the lower limits to avoid nasty bugs. But AFAICS, a limit
of SOCK_MIN_SNDBUF isn't necessary to do that: the BUG_ON cited in the
commit message seems to have happened because unix_stream_sendmsg()
expects a minimum of a full page (ie SK_MEM_QUANTUM) and the math broke,
not because it had less than SOCK_MIN_SNDBUF allocated.

This particular issue doesn't seem to affect TCP however: using a
setting of "1 1 1" for tcp_{r,w}mem works, although it's obviously
suboptimal. SK_MEM_QUANTUM would be a nice minimum, but it's 64K on
some archs, so there would still be breakage.

Since a value of one doesn't seem to cause any problems, we can drop the
minimum 8133534c added to fix this.

This reverts commit 8133534c760d4083f79d2cde42c636ccc0b2792e.

Fixes: 8133534c760d4083 ("net: limit tcp/udp rmem/wmem to SOCK_MIN...")
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: Sorin Dumitru <sorin@...urnze.ro>
Signed-off-by: Calvin Owens <calvinowens@...com>
---
 net/ipv4/sysctl_net_ipv4.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 433231c..0330ab2 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -41,8 +41,6 @@ static int tcp_syn_retries_min = 1;
 static int tcp_syn_retries_max = MAX_TCP_SYNCNT;
 static int ip_ping_group_range_min[] = { 0, 0 };
 static int ip_ping_group_range_max[] = { GID_T_MAX, GID_T_MAX };
-static int min_sndbuf = SOCK_MIN_SNDBUF;
-static int min_rcvbuf = SOCK_MIN_RCVBUF;

 /* Update system visible IP port range */
 static void set_local_port_range(struct net *net, int range[2])
@@ -530,7 +528,7 @@ static struct ctl_table ipv4_table[] = {
 		.maxlen		= sizeof(sysctl_tcp_wmem),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
-		.extra1		= &min_sndbuf,
+		.extra1		= &one,
 	},
 	{
 		.procname	= "tcp_notsent_lowat",
@@ -545,7 +543,7 @@ static struct ctl_table ipv4_table[] = {
 		.maxlen		= sizeof(sysctl_tcp_rmem),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
-		.extra1		= &min_rcvbuf,
+		.extra1		= &one,
 	},
 	{
 		.procname	= "tcp_app_win",
@@ -758,7 +756,7 @@ static struct ctl_table ipv4_table[] = {
 		.maxlen		= sizeof(sysctl_udp_rmem_min),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
-		.extra1		= &min_rcvbuf,
+		.extra1		= &one
 	},
 	{
 		.procname	= "udp_wmem_min",
@@ -766,7 +764,7 @@ static struct ctl_table ipv4_table[] = {
 		.maxlen		= sizeof(sysctl_udp_wmem_min),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
-		.extra1		= &min_sndbuf,
+		.extra1		= &one
 	},
 	{ }
 };
-- 
2.5.0

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