GitLab

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 ab...