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

On 2026-03-20 09:31, Mathieu Desnoyers wrote:

On 2026-03-20 09:21, Harry Yoo (Oracle) wrote:

quoted

On Fri, Mar 20, 2026 at 08:35:46AM -0400, Mathieu Desnoyers wrote:

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On 2026-03-20 00:17, Harry Yoo wrote:
[...]

quoted

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[1]: https://lore.kernel.org/20260227153730.1556542-4- mathieu.desnoyers@efficios.com/

@Mathieu: In patch 1/3 description,

quoted

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:

[...]

One thing we could do to catch this kind of init sequence issue
is to add a WARN_ON_ONCE in percpu_counter_tree_items_size:

size_t percpu_counter_tree_items_size(void)
{
        if (WARN_ON_ONCE(!nr_cpus_order))
                return 0;
        return counter_config->nr_items * sizeof(struct percpu_counter_tree_level_item);
}

Thanks,

Mathieu

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