On Mon, Sep 23, 2019 at 05:28:56PM +0200, Michal Hocko wrote:

On Mon 23-09-19 17:15:19, Peter Zijlstra wrote:

quoted

quoted

diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
index 4123100e..9859acb 100644
--- a/arch/x86/mm/numa.c
+++ b/arch/x86/mm/numa.c

@@ -861,6 +861,9 @@ void numa_remove_cpu(int cpu)
  */
 const struct cpumask *cpumask_of_node(int node)
 {
+	if (node == NUMA_NO_NODE)
+		return cpu_online_mask;

This mandates the caller holds cpus_read_lock() or something, I'm pretty
sure that if I put:

	lockdep_assert_cpus_held();

Is this documented somewhere?

No idea... common sense :-)

Also how does that differ from a normal
case when a proper node is used? The cpumask will always be dynamic in
the cpu hotplug presence, right?

As per normal yes, and I'm fairly sure there's a ton of bugs. Any
'online' state is subject to change except when you're holding
sufficient locks to stop it.

Disabling preemption also stabilizes it, because cpu unplug relies on
stop-machine.

quoted

here, it comes apart real quick. Without holding the cpu hotplug lock,
the online mask is gibberish.

Can the returned cpu mask go away?

No, the cpu_online_mask itself has static storage, the contents OTOH can
change at will. Very little practical difference :-)