Re: NULL pointer dereference when booting ppc64_guest_defconfig in QEMU on -next

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-03-20 00:17, Harry Yoo wrote:
[...]
>> [1]: https://lore.kernel.org/20260227153730.1556542-4-mathieu.desnoyers@efficios.com/
> 
> @Mathieu: In patch 1/3 description,
>> Changes since v7:
>> - Explicitly initialize the subsystem from start_kernel() right
>>    after mm_core_init() so it is up and running before the creation of
>>    the first mm at boot.
> 
> But how does this work when someone calls mm_cpumask() on init_mm early?
> Looks like it will behave incorrectly because get_rss_stat_items_size()
> returns zero?

It doesn't work as expected at all. I missed that all users of mm_cpumask()
end up relying on get_rss_stat_items_size(), which now calls
percpu_counter_tree_items_size(), which depends on initialization from
percpu_counter_tree_subsystem_init().

If you add a call to percpu_counter_tree_subsystem_init in
arch/powerpc/kernel/setup_arch() just before:

         VM_WARN_ON(cpumask_test_cpu(smp_processor_id(), mm_cpumask(&init_mm)));
         cpumask_set_cpu(smp_processor_id(), mm_cpumask(&init_mm));

Does the warning go away ?

Alternatively, would could use a lazy initialization invoking
percpu_counter_tree_subsystem_init from percpu_counter_tree_items_size
when the initialization is not already done.

Any preference ?

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com