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

[Christoph Hellwig]

On Thu, Jan 26, 2023 at 10:51:17AM -0800, Dave Hansen wrote:
> 
> Also, what are the practical implications here?  There are obviously an
> ever decreasing number of 32-bit DMA devices out there.  Somebody that
> has one and uses this option might be sad because now they're stuck
> using ZONE_DMA which is quite tiny.
> 
> What other ZONE_DMA32 users are left?  Will anyone else care?  There is
> some DMA32 slab and vmalloc() functionality remaining.  Is it impacted?

DMA32 never supported lab.  But < 64-bit DMA device are unfortunately
still not uncommon, and configuring out ZONE_DMA32 breaks them pretty
badly as we guarantee that a DMA mask of 32-bit always works.

So I'm not only very much against this patch, but also the currently
existing way to configure out ZONE_DMA32 on arm64, which needs to
go away.

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.