Subject: Re: [5.6.0-rc2-next-20200218/powerpc] Boot failure on POWER9

From: Michael Ellerman <[email-masked]>
Date: Thu, 12 Mar 2020 23:18:42 +1100

Michal Hocko <[email-masked]> writes:
> On Thu 27-02-20 19:26:54, Michal Hocko wrote:
>> [Cc ppc maintainers]
> [...]
>> Please have a look at http://lkml.kernel.org/r/[email-masked]
>> for the boot log with the debugging patch which tracks set_numa_mem.
>> This seems to lead to a crash in the slab allocator bebcause
>> node_to_mem_node(0) for memory less node resolves to the memory less
>> node http://lkml.kernel.org/r/[email-masked].
>> The original report is http://lkml.kernel.org/r/[email-masked]
>
> ping 

The obvious fix is:

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 37c12e3bab9e..33b1fca0b258 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -892,6 +892,7 @@ void smp_prepare_boot_cpu(void)
 	paca_ptrs[boot_cpuid]->__current = current;
 #endif
 	set_numa_node(numa_cpu_lookup_table[boot_cpuid]);
+	set_numa_mem(local_memory_node(numa_cpu_lookup_table[boot_cpuid]));
 	current_set[boot_cpuid] = current;
 }

But that doesn't work because smp_prepare_boot_cpu() is called too
early:

asmlinkage __visible void __init start_kernel(void)
{
	...
	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
	boot_cpu_hotplug_init();

	build_all_zonelists(NULL);

And local_memory_node() uses first_zones_zonelist() which doesn't work
prior to build_all_zonelists() being called.

The patch below might work. Sachin can you test this? I tried faking up
a system with a memoryless node zero but couldn't get it to even start
booting.

cheers

diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
index 9b4f5fb719e0..d1f11437f6c4 100644
--- a/arch/powerpc/mm/mem.c
+++ b/arch/powerpc/mm/mem.c
@@ -282,6 +282,9 @@ void __init mem_init(void)
 	 */
 	BUILD_BUG_ON(MMU_PAGE_COUNT > 16);
 
+	BUG_ON(smp_processor_id() != boot_cpuid);
+	set_numa_mem(local_memory_node(numa_cpu_lookup_table[boot_cpuid]));
+
 #ifdef CONFIG_SWIOTLB
 	/*
 	 * Some platforms (e.g. 85xx) limit DMA-able memory way below