Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA

[Ard Biesheuvel]

On 4 January 2017 at 13:28, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Wed, Dec 14, 2016 at 09:11:47AM +0000, Ard Biesheuvel wrote:
>> The NUMA code may get confused by the presence of NOMAP regions within
>> zones, resulting in spurious BUG() checks where the node id deviates
>> from the containing zone's node id.
>>
>> Since the kernel has no business reasoning about node ids of pages it
>> does not own in the first place, enable CONFIG_HOLES_IN_ZONE to ensure
>> that such pages are disregarded.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>> ---
>>  arch/arm64/Kconfig | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 111742126897..0472afe64d55 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -614,6 +614,10 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
>>       def_bool y
>>       depends on NUMA
>>
>> +config HOLES_IN_ZONE
>> +     def_bool y
>> +     depends on NUMA
>> +
>>  source kernel/Kconfig.preempt
>>  source kernel/Kconfig.hz
>
> I'm happy to apply this, but I'll hold off until the first patch is queued
> somewhere, since this doesn't help without the VM_BUG_ON being moved.
>
> Alternatively, I can queue both if somebody from the mm camp acks the
> first patch.
>

Actually, I am not convinced the discussion is finalized. These
patches do fix the issue, but Robert also suggested an alternative fix
which may be preferable.

http://marc.info/?l=linux-arm-kernel&m=148190753510107&w=2

I haven't responded to it yet, due to the holidays, but I'd like to
explore that solution a bit further before applying anything, if you
don't mind.

Thanks,
Ard.