tcp_memcontrol: Cleanup/fix cg_proto->memory_pressure handling.

kill memcg_tcp_enter_memory_pressure.  The only function of
memcg_tcp_enter_memory_pressure was to reduce deal with the
unnecessary abstraction that was tcp_memcontrol.  Now that struct
tcp_memcontrol is gone remove this unnecessary function, the
unnecessary function pointer, and modify sk_enter_memory_pressure to
set this field directly, just as sk_leave_memory_pressure cleas this
field directly.

This fixes a small bug I intruduced when killing struct tcp_memcontrol
that caused memcg_tcp_enter_memory_pressure to never be called and
thus failed to ever set cg_proto->memory_pressure.

Remove the cg_proto enter_memory_pressure function as it now serves
no useful purpose.

Don't test cg_proto->memory_presser in sk_leave_memory_pressure before
clearing it.  The test was originally there to ensure that the pointer
was non-NULL.  Now that cg_proto is not a pointer the pointer does not
matter.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1035,7 +1035,6 @@ enum cg_proto_flags {
 };
 
 struct cg_proto {
-	void			(*enter_memory_pressure)(struct sock *sk);
 	struct res_counter	memory_allocated;	/* Current allocated memory. */
 	struct percpu_counter	sockets_allocated;	/* Current number of sockets. */
 	int			memory_pressure;
@@ -1155,8 +1154,7 @@ static inline void sk_leave_memory_pressure(struct sock *sk)
 		struct proto *prot = sk->sk_prot;
 
 		for (; cg_proto; cg_proto = parent_cg_proto(prot, cg_proto))
-			if (cg_proto->memory_pressure)
-				cg_proto->memory_pressure = 0;
+			cg_proto->memory_pressure = 0;
 	}
 
 }
@@ -1171,7 +1169,7 @@ static inline void sk_enter_memory_pressure(struct sock *sk)
 		struct proto *prot = sk->sk_prot;
 
 		for (; cg_proto; cg_proto = parent_cg_proto(prot, cg_proto))
-			cg_proto->enter_memory_pressure(sk);
+			cg_proto->memory_pressure = 1;
 	}
 
 	sk->sk_prot->enter_memory_pressure(sk);

net/ipv4/tcp_memcontrol.c

@@ -6,13 +6,6 @@
 #include <linux/memcontrol.h>
 #include <linux/module.h>
 
-static void memcg_tcp_enter_memory_pressure(struct sock *sk)
-{
-	if (sk->sk_cgrp->memory_pressure)
-		sk->sk_cgrp->memory_pressure = 1;
-}
-EXPORT_SYMBOL(memcg_tcp_enter_memory_pressure);
-
 int tcp_init_cgroup(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
 {
 	/*