RE: [PATCH v3] lib/group_cpus: Snapshot cluster masks to keep grouping hotplug invariant

+	for_each_possible_cpu(cpu) {
+		if (!zalloc_cpumask_var(&snapshot[cpu], GFP_KERNEL)) {
+			while (cpu--)
for_each_possible_cpu(cpu) may have holes, but "while (cpu--)" assume the enumeration is dense.
Although freeing NULL is not a bug, it worths a comment or check for such subtle thing. 
+				free_cpumask_var(snapshot[cpu]);
+			kfree(snapshot);
+			goto out;
+		}
+	}