From 558f86493df09f68f79fe056d9028d317a3ce8ab Mon Sep 17 00:00:00 2001
From: Nathan Lynch <nathanl@linux.ibm.com>
Date: Thu, 18 Apr 2019 13:56:58 -0500
Subject: [PATCH] powerpc/numa: document topology_updates_enabled, disable by
 default

References: bsc#1133584
Patch-mainline: v5.2-rc1
Git-commit: 558f86493df09f68f79fe056d9028d317a3ce8ab

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: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Michal Suchanek <msuchanek@suse.de>
---
 arch/powerpc/mm/numa.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index 952ada44df62..6ef36d553cde 100644
--- a/arch/powerpc/mm/numa.c
+++ b/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;
-- 
2.20.1