Re: [RFC PATCH] mm: CONFIG_NR_ZONES_EXTENDED

[Vlastimil Babka]

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?

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