Re: [RFC PATCH] mm: CONFIG_NR_ZONES_EXTENDED

[Dan Williams]

On Tue, Mar 1, 2016 at 12:31 AM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 03/01/2016 03:06 AM, Dan Williams wrote:
>>
>> On Mon, Feb 29, 2016 at 4:06 PM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
>>>
>>> On 29.2.2016 18:55, Dan Williams wrote:
>>>>
>>>> On Mon, Feb 29, 2016 at 4:33 AM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
>>>>>
>>>>> On 02/02/2016 06:42 AM, Andrew Morton wrote:
>>>>
>>>>
>>>> In this case it's already part of the equation because:
>>>>
>>>> config ZONE_DEVICE
>>>>        depends on MEMORY_HOTPLUG
>>>>        depends on MEMORY_HOTREMOVE
>>>>
>>>> ...and those in turn depend on SPARSEMEM.
>>>
>>>
>>> Fine, but then SPARSEMEM_VMEMMAP should be still an available subvariant
>>> of
>>> SPARSEMEM with SECTION_WIDTH=0.
>>
>>
>> It should be, but not for the ZONE_DEVICE case.  ZONE_DEVICE depends
>> on x86_64 which means ZONE_DEVICE also implies SPARSEMEM_VMEMMAP
>> since:
>>
>> config ARCH_SPARSEMEM_ENABLE
>>         def_bool y
>>         depends on X86_64 || NUMA || X86_32 || X86_32_NON_STANDARD
>>         select SPARSEMEM_STATIC if X86_32
>>         select SPARSEMEM_VMEMMAP_ENABLE if X86_64
>>
>> Now, if a future patch wants to reclaim page flags space for other
>> usages outside of ZONE_DEVICE it can do the work to handle the
>> SPARSEMEM_VMEMMAP=n case.  I don't see a reason to fold that
>> distinction into the current patch given the current constraints.
>
>
> OK so that IUUC shows that x86_64 should be always fine without decreasing
> the range for NODES_SHIFT? That's basically my point - since there's a
> configuration where things don't fit (32bit?), the patch broadly decreases
> range for NODES_SHIFT for everyone, right?

So I went hunting for the x86_64 config that sent me off in this
direction in the first place, but I can't reproduce it.  I'm indeed
able to fit ZONE_DEVICE + ZONE_DMA + NODES_SHIFT(10) without
overflowing page flags.  Maybe we reduced some usage page->flags usage
between 4.3 and 4.5 and I missed it?

In any event, you're right we can indeed fit ZONE_DEVICE into the
current MAXSMP definition.  I'll respin the patch.

Thanks for probing on this!