BugLink: http://bugs.launchpad.net/bugs/1534647

ZONE_DEVICE (merged in 4.3) and ZONE_CMA (proposed) are examples of new mm
zones that are bumping up against the current maximum limit of 4 zones,
i.e.  2 bits in page->flags.  When adding a zone this equation still needs
to be satisified:

    SECTIONS_WIDTH + ZONES_WIDTH + NODES_SHIFT + LAST_CPUPID_SHIFT
	  <= BITS_PER_LONG - NR_PAGEFLAGS

ZONE_DEVICE currently tries to satisfy this equation by requiring that
ZONE_DMA be disabled, but this is untenable given generic kernels want to
support ZONE_DEVICE and ZONE_DMA simultaneously.  ZONE_CMA would like to
increase the amount of memory covered per section, but that limits the
minimum granularity at which consecutive memory ranges can be added via
devm_memremap_pages().

The trade-off of what is acceptable to sacrifice depends heavily on the
platform.  For example, ZONE_CMA is targeted for 32-bit platforms where
page->flags is constrained, but those platforms likely do not care about
the minimum granularity of memory hotplug.  A big iron machine with 1024
numa nodes can likely sacrifice ZONE_DMA where a general purpose
distribution kernel can not.

CONFIG_NR_ZONES_EXTENDED is a configuration symbol that gets selected when
the number of configured zones exceeds 4.  It documents the configuration
symbols and definitions that get modified when ZONES_WIDTH is greater than
2.

For now, it steals a bit from NODES_SHIFT.  Later on it can be used to
document the definitions that get modified when a 32-bit configuration
wants more zone bits.

Note that GFP_ZONE_TABLE poses an interesting constraint since
include/linux/gfp.h gets included by the 32-bit portion of a 64-bit build.
We need to be careful to only build the table for zones that have a
corresponding gfp_t flag.  GFP_ZONES_SHIFT is introduced for this purpose.
This patch does not attempt to solve the problem of adding a new zone
that also has a corresponding GFP_ flag.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=110931
Fixes: 033fbae9 ("mm: ZONE_DEVICE for "device memory"")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reported-by: Mark <markk@clara.co.uk>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from linux-next commit 27ffb3827ac71a46e8d52fc7ed7302d33a619d6c)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>