[RFC] mm: Allow ZONE_DMA32 to be disabled via kernel command line

[Christoph Hellwig]

On Thu, Jan 26, 2023 at 10:52:43PM -0800, H. Peter Anvin wrote:
> >If people want ZONE_DMA32 to go away we need something to replace
> >it first, like a large enough CMA region in the 32-bit addressable
> >range.
> 
> Not to mention all kinds of odd masks like 30, 31, 39, 40, 46, ... bits.

Yes.  Out of those all >= 32 are falling straight into ZONE_DM32,
the lower ones we do a first try in ZONE_DMA32 and then fall back to
ZONE_DMA.  <= 29 mask OTOH are really rate in modern systems for
actual devices.  So with a CMA region for what is currently ZONE_DMA
and one for the first 1G we'd probably cover most of what's actually
needed for x86_64.  Of course on 32-bit architetures things become
a lot more complicated due to highmem.