powerpc/numa: document topology_updates_enabled, disable by default

Changing the NUMA associations for CPUs and memory at runtime is
basically unsupported by the core mm, scheduler etc. We see all manner
of crashes, warnings and instability when the pseries code tries to do
this. Disable this behavior by default, and document the switch a bit.
Signed-off-by: NNathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/numa.c

@@ -907,16 +907,22 @@ static int __init early_numa(char *p)
 }
 early_param("numa", early_numa);
 
-static bool topology_updates_enabled = true;
+/*
+ * The platform can inform us through one of several mechanisms
+ * (post-migration device tree updates, PRRN or VPHN) that the NUMA
+ * assignment of a resource has changed. This controls whether we act
+ * on that. Disabled by default.
+ */
+static bool topology_updates_enabled;
 
 static int __init early_topology_updates(char *p)
 {
 	if (!p)
 		return 0;
 
-	if (!strcmp(p, "off")) {
-		pr_info("Disabling topology updates\n");
-		topology_updates_enabled = false;
+	if (!strcmp(p, "on")) {
+		pr_warn("Caution: enabling topology updates\n");
+		topology_updates_enabled = true;
 	}
 
 	return 0;