powerpc: Account mm_cpumask and active_cpus in init_mm

init_mm mm_cpumask and context.active_cpus is not maintained at boot
and hotplug. This seems to be harmless because init_mm does not have a
userspace and so never gets user TLBs flushed, but it looks odd and it
prevents some sanity checks being added.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230524060821.148015-2-npiggin@gmail.com

arch/powerpc/kernel/setup-common.c

@@ -969,8 +969,12 @@ void __init setup_arch(char **cmdline_p)
 	klp_init_thread_info(&init_task);
 
 	setup_initial_init_mm(_stext, _etext, _edata, _end);
-
+	/* sched_init() does the mmgrab(&init_mm) for the primary CPU */
+	VM_WARN_ON(cpumask_test_cpu(smp_processor_id(), mm_cpumask(&init_mm)));
+	cpumask_set_cpu(smp_processor_id(), mm_cpumask(&init_mm));
+	inc_mm_active_cpus(&init_mm);
 	mm_iommu_init(&init_mm);
+
 	irqstack_early_init();
 	exc_lvl_early_init();
 	emergency_stack_init();

arch/powerpc/kernel/smp.c

@@ -47,6 +47,7 @@
 #include <asm/smp.h>
 #include <asm/time.h>
 #include <asm/machdep.h>
+#include <asm/mmu_context.h>
 #include <asm/cputhreads.h>
 #include <asm/cputable.h>
 #include <asm/mpic.h>
@@ -1616,6 +1617,9 @@ void start_secondary(void *unused)
 
 	mmgrab_lazy_tlb(&init_mm);
 	current->active_mm = &init_mm;
+	VM_WARN_ON(cpumask_test_cpu(smp_processor_id(), mm_cpumask(&init_mm)));
+	cpumask_set_cpu(cpu, mm_cpumask(&init_mm));
+	inc_mm_active_cpus(&init_mm);
 
 	smp_store_cpu_info(cpu);
 	set_dec(tb_ticks_per_jiffy);
@@ -1751,6 +1755,14 @@ int __cpu_disable(void)
 
 void __cpu_die(unsigned int cpu)
 {
+	/*
+	 * This could perhaps be a generic call in idlea_task_dead(), but
+	 * that requires testing from all archs, so first put it here to
+	 */
+	VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu, mm_cpumask(&init_mm)));
+	dec_mm_active_cpus(&init_mm);
+	cpumask_clear_cpu(cpu, mm_cpumask(&init_mm));
+
 	if (smp_ops->cpu_die)
 		smp_ops->cpu_die(cpu);
 }

arch/powerpc/mm/mmu_context.c

@@ -47,6 +47,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 
 	/* Mark this context has been used on the new CPU */
 	if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(next))) {
+		VM_WARN_ON_ONCE(next == &init_mm);
 		cpumask_set_cpu(smp_processor_id(), mm_cpumask(next));
 		inc_mm_active_cpus(next);