On Tue, Mar 1, 2016 at 12:31 AM, Vlastimil Babka [off-list ref] wrote:

On 03/01/2016 03:06 AM, Dan Williams wrote:

quoted

On Mon, Feb 29, 2016 at 4:06 PM, Vlastimil Babka [off-list ref] wrote:

quoted

On 29.2.2016 18:55, Dan Williams wrote:

quoted

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:

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!

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