[PATCH] powerpc/numa: fix hot-added CPU on memory-less node

[Laurent Vivier]

Trying to hotplug a CPU on an empty NUMA node (without
memory or CPU) crashes the kernel when the CPU is onlined.

During the onlining process, the kernel calls start_secondary()
that ends by calling
set_numa_mem(local_memory_node(numa_cpu_lookup_table[cpu]))
that relies on NODE_DATA(nid)->node_zonelists and in our case
NODE_DATA(nid) is NULL.

To fix that, add the same checking as we already have in
find_and_online_cpu_nid(): if NODE_DATA() is NULL, use
the first online node.

Bug: https://github.com/linuxppc/linux/issues/184
Fixes: ea05ba7c559c8e5a5946c3a94a2a266e9a6680a6
       (powerpc/numa: Ensure nodes initialized for hotplug)
Signed-off-by: Laurent Vivier <lvivier@xxxxxxxxxx>
---
 arch/powerpc/mm/numa.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index 3a048e98a132..1b2d25a3c984 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c
@@ -483,6 +483,15 @@ static int numa_setup_cpu(unsigned long lcpu)
 	if (nid < 0 || !node_possible(nid))
 		nid = first_online_node;
 
+	if (NODE_DATA(nid) == NULL) {
+		/*
+		 * Default to using the nearest node that has memory installed.
+		 * Otherwise, it would be necessary to patch the kernel MM code
+		 * to deal with more memoryless-node error conditions.
+		 */
+		nid = first_online_node;
+	}
+
 	map_cpu_to_node(lcpu, nid);
 	of_node_put(cpu);
 out:
-- 
2.17.2