net: sctp: dynamically enable or disable pf state

As we all know, the value of pf_retrans >= max_retrans_path can
disable pf state. The variables of pf_retrans and max_retrans_path
can be changed by the userspace application.

Sometimes the user expects to disable pf state while the 2
variables are changed to enable pf state. So it is necessary to
introduce a new variable to disable pf state.

According to the suggestions from Vlad Yasevich, extra1 and extra2
are removed. The initialization of pf_enable is added.

Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: Zhu Yanjun <zyjzyj2000@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 2ea4c45..5de632e 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -1723,6 +1723,25 @@
 
 	Default: 0
 
+pf_enable - INTEGER
+	Enable or disable pf (pf is short for potentially failed) state. A value
+	of pf_retrans > path_max_retrans also disables pf state. That is, one of
+	both pf_enable and pf_retrans > path_max_retrans can disable pf state.
+	Since pf_retrans and path_max_retrans can be changed by userspace
+	application, sometimes user expects to disable pf state by the value of
+	pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
+	or path_max_retrans is changed by the user application, this pf state is
+	enabled. As such, it is necessary to add this to dynamically enable
+	and disable pf state. See:
+	https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for
+	details.
+
+	1: Enable pf.
+
+	0: Disable pf.
+
+	Default: 1
+
 addip_noauth_enable - BOOLEAN
 	Dynamic Address Reconfiguration (ADD-IP) requires the use of
 	authentication to protect the operations of adding or removing new
@@ -1799,7 +1818,9 @@
 	having to reduce path_max_retrans to a very low value.  See:
 	http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
 	for details.  Note also that a value of pf_retrans > path_max_retrans
-	disables this feature
+	disables this feature. Since both pf_retrans and path_max_retrans can
+	be changed by userspace application, a variable pf_enable is used to
+	disable pf state.
 
 	Default: 0