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Date: Tue, 26 Jan 2016 14:51:53 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Rik van Riel <riel@redhat.com>, linux-nvdimm <linux-nvdimm@ml01.01.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Ingo Molnar <mingo@redhat.com>, Mel Gorman <mgorman@suse.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Jerome Glisse <j.glisse@gmail.com>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Subject: Re: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE_DMA
Message-Id: <20160126145153.44e4f38b04200209d133c0a3@linux-foundation.org>
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On Tue, 26 Jan 2016 14:33:48 -0800 Dan Williams <dan.j.williams@intel.com> 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?