x86-64, NUMA: Better explain numa_distance handling

Handling of out-of-bounds distances and allocation failure can use
better documentation.  Add it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
diff --git a/arch/x86/mm/numa_emulation.c b/arch/x86/mm/numa_emulation.c
index 0afa25d..aeecea9 100644
--- a/arch/x86/mm/numa_emulation.c
+++ b/arch/x86/mm/numa_emulation.c
@@ -379,7 +379,11 @@
 		if (emu_nid_to_phys[i] == NUMA_NO_NODE)
 			emu_nid_to_phys[i] = 0;
 
-	/* transform distance table */
+	/*
+	 * Transform distance table.  numa_set_distance() ignores all
+	 * out-of-bound distances.  Just call it for every possible node
+	 * combination.
+	 */
 	numa_reset_distance();
 	for (i = 0; i < MAX_NUMNODES; i++) {
 		for (j = 0; j < MAX_NUMNODES; j++) {