sock: Doc behaviors for pressure heurisitics

There are now two accounting infrastructures for skmem, while the
heuristics in __sk_mem_raise_allocated() were actually introduced
before memcg was born.

Add some comments to clarify whether they can be applied to both
infrastructures or not.
Suggested-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231019120026.42215-2-wuyun.abel@bytedance.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/sock.c

@@ -3067,7 +3067,14 @@ int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
 	if (allocated > sk_prot_mem_limits(sk, 2))
 		goto suppress_allocation;
 
-	/* guarantee minimum buffer size under pressure */
+	/* Guarantee minimum buffer size under pressure (either global
+	 * or memcg) to make sure features described in RFC 7323 (TCP
+	 * Extensions for High Performance) work properly.
+	 *
+	 * This rule does NOT stand when exceeds global or memcg's hard
+	 * limit, or else a DoS attack can be taken place by spawning
+	 * lots of sockets whose usage are under minimum buffer size.
+	 */
 	if (kind == SK_MEM_RECV) {
 		if (atomic_read(&sk->sk_rmem_alloc) < sk_get_rmem0(sk, prot))
 			return 1;
@@ -3088,6 +3095,11 @@ int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
 
 		if (!sk_under_memory_pressure(sk))
 			return 1;
+
+		/* Try to be fair among all the sockets under global
+		 * pressure by allowing the ones that below average
+		 * usage to raise.
+		 */
 		alloc = sk_sockets_allocated_read_positive(sk);
 		if (sk_prot_mem_limits(sk, 2) > alloc *
 		    sk_mem_pages(sk->sk_wmem_queued +