Re: [RFC PATCH] mm: CONFIG_NR_ZONES_EXTENDED

[Vlastimil Babka]

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:
>>> So if you want ZONE_DMA, you're limited to 512 NUMA nodes?
>>>
>>> That seems reasonable.
>>
>>
>> Sorry for the late reply, but it seems that with !SPARSEMEM, or with
>> SPARSEMEM_VMEMMAP, reducing NUMA nodes isn't even necessary, because
>> SECTIONS_WIDTH is zero (see the diagrams in linux/page-flags-layout.h). In
>> my brief tests with 4.4 based kernel with SPARSEMEM_VMEMMAP it seems that
>> with 1024 NUMA nodes and 8192 CPU's, there's still 7 bits left (i.e. 6 with
>> CONFIG_NR_ZONES_EXTENDED).
>>
>> With the danger of becoming even more complex, could the limit also depend
>> on CONFIG_SPARSEMEM/VMEMMAP to reflect that somehow?
> 
> 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.