[PATCH v4] powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt.

[Aneesh Kumar K V]

On 3/7/24 5:13 PM, Michael Ellerman wrote:
> Hi Mahesh,
> 
> Mahesh Salgaonkar <mahesh at linux.ibm.com> writes:
>> nmi_enter()/nmi_exit() touches per cpu variables which can lead to kernel
>> crash when invoked during real mode interrupt handling (e.g. early HMI/MCE
>> interrupt handler) if percpu allocation comes from vmalloc area.
>>
>> Early HMI/MCE handlers are called through DEFINE_INTERRUPT_HANDLER_NMI()
>> wrapper which invokes nmi_enter/nmi_exit calls. We don't see any issue when
>> percpu allocation is from the embedded first chunk. However with
>> CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled there are chances where percpu
>> allocation can come from the vmalloc area.
>>
>> With kernel command line "percpu_alloc=page" we can force percpu allocation
>> to come from vmalloc area and can see kernel crash in machine_check_early:
>>
>> [    1.215714] NIP [c000000000e49eb4] rcu_nmi_enter+0x24/0x110
>> [    1.215717] LR [c0000000000461a0] machine_check_early+0xf0/0x2c0
>> [    1.215719] --- interrupt: 200
>> [    1.215720] [c000000fffd73180] [0000000000000000] 0x0 (unreliable)
>> [    1.215722] [c000000fffd731b0] [0000000000000000] 0x0
>> [    1.215724] [c000000fffd73210] [c000000000008364] machine_check_early_common+0x134/0x1f8
>>
>> Fix this by avoiding use of nmi_enter()/nmi_exit() in real mode if percpu
>> first chunk is not embedded.
> 
> My system (powernv) doesn't even boot with percpu_alloc=page.
> 


Can you share the crash details? 


> AFAIK the only reason we added support for it was to handle 4K kernels
> with HPT. See commit eb553f16973a ("powerpc/64/mm: implement page
> mapping percpu first chunk allocator").
> 
> So I wonder if we should change the Kconfig to only offer it as an
> option in that case, and change the logic in setup_per_cpu_areas() to
> only use it as a last resort.
> 
> I guess we probably still need this commit though, even if just for 4K HPT.
> 
>
We have also observed some error when we have large gap between the start memory of
NUMA nodes. That made the percpu offset really large causing boot failures even on 64K.

-aneesh