On 29.2.2016 18:55, Dan Williams wrote:

On Mon, Feb 29, 2016 at 4:33 AM, Vlastimil Babka [off-list ref] wrote:

quoted

On 02/02/2016 06:42 AM, Andrew Morton wrote:

quoted

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.

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