Re: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE_DMA

[Dan Williams]

On Tue, Jan 26, 2016 at 2:51 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 26 Jan 2016 14:33:48 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
>> >> Towards this end, alias ZONE_DMA and ZONE_DEVICE to work around needing
>> >> to maintain a unique zone number for ZONE_DEVICE.  Record the geometry
>> >> of ZONE_DMA at init (->init_spanned_pages) and use that information in
>> >> is_zone_device_page() to differentiate pages allocated via
>> >> devm_memremap_pages() vs true ZONE_DMA pages.  Otherwise, use the
>> >> simpler definition of is_zone_device_page() when ZONE_DMA is turned off.
>> >>
>> >> Note that this also teaches the memory hot remove path that the zone may
>> >> not have sections for all pfn spans (->zone_dyn_start_pfn).
>> >>
>> >> A user visible implication of this change is potentially an unexpectedly
>> >> high "spanned" value in /proc/zoneinfo for the DMA zone.
>> >
>> > Well, all these icky tricks are to avoid increasing ZONES_SHIFT, yes?
>> > Is it possible to just use ZONES_SHIFT=3?
>>
>> Last I tried I hit this warning in mm/memory.c
>>
>> #warning Unfortunate NUMA and NUMA Balancing config, growing
>> page-frame for last_cpupid.
>
> Well yes, it may take a bit of work - perhaps salvaging a bit from
> somewhere else if poss.  But that might provide a better overall
> solution so could you please have a think?
>

Will do, especially since other efforts are feeling the pinch on the
MAX_NR_ZONES limitation.