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SubjectRe: [PATCH] [8/13] Enable the mask allocator for x86
On Fri, 7 Mar 2008, Andi Kleen wrote:

> - Disable old ZONE_DMA
> No fixed size ZONE_DMA now anymore. All existing users of __GFP_DMA rely
> on the compat call to the maskable allocator in alloc/free_pages
> - Call maskable allocator initialization functions at boot
> - Add TRAD_DMA_MASK for the compat functions
> - Remove dma_reserve call

This looks okay for the disabling part. But note that there are various
uses of MAX_DMA_ADDRESS (sparsemem, bootmem allocator) that are currently
suboptimal because they set a boundary at 16MB for allocation of
potentially large operating system structures. That boundary continues to
exist despite the removal of ZONE_DMA?

It would be better to remove ZONE_DMA32 instead and enlarge ZONE_DMA so
that it can take over the role of ZONE_DMA. Set the boundary for
MAX_DMA_ADDRESS to the boundary for ZONE_DMA32. Then the
allocations for sparse and bootmem will be allocated above 4GB which
leaves lots of the lower space available for 32 bit DMA capable devices.

Removal of ZONE_DMA32 would allow us to remove support for that zone from
the kernel in general since x86_64 is the only user of that zone.

Trouble is likely that there are existing users that have the expectation
of a 4GB boundary on ZONE_DMA32 and 16MB on ZONE_DMA (particularly old
drivers). Could we not clean that up?

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