sock: Fix misuse of sk_under_memory_pressure()

The status of global socket memory pressure is updated when:

  a) __sk_mem_raise_allocated():

	enter: sk_memory_allocated(sk) >  sysctl_mem[1]
	leave: sk_memory_allocated(sk) <= sysctl_mem[0]

  b) __sk_mem_reduce_allocated():

	leave: sk_under_memory_pressure(sk) &&
		sk_memory_allocated(sk) < sysctl_mem[0]

So the conditions of leaving global pressure are inconstant, which
may lead to the situation that one pressured net-memcg prevents the
global pressure from being cleared when there is indeed no global
pressure, thus the global constrains are still in effect unexpectedly
on the other sockets.

This patch fixes this by ignoring the net-memcg's pressure when
deciding whether should leave global memory pressure.

Fixes: e1aab16 ("socket: initial cgroup code.")
Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Link: https://lore.kernel.org/r/20230816091226.1542-1-wuyun.abel@bytedance.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Abel-WY

include/net/sock.h

@@ -1420,6 +1420,12 @@ static inline bool sk_has_memory_pressure(const struct sock *sk)
 	return sk->sk_prot->memory_pressure != NULL;
 }
 
+static inline bool sk_under_global_memory_pressure(const struct sock *sk)
+{
+	return sk->sk_prot->memory_pressure &&
+		!!*sk->sk_prot->memory_pressure;
+}
+
 static inline bool sk_under_memory_pressure(const struct sock *sk)
 {
 	if (!sk->sk_prot->memory_pressure)

net/core/sock.c

@@ -3159,7 +3159,7 @@ void __sk_mem_reduce_allocated(struct sock *sk, int amount)
 	if (mem_cgroup_sockets_enabled && sk->sk_memcg)
 		mem_cgroup_uncharge_skmem(sk->sk_memcg, amount);
 
-	if (sk_under_memory_pressure(sk) &&
+	if (sk_under_global_memory_pressure(sk) &&
 	    (sk_memory_allocated(sk) < sk_prot_mem_limits(sk, 0)))
 		sk_leave_memory_pressure(sk);
 }