Re: [patch] x86: allow ZONE_DMA to be configurable

[H. Peter Anvin]

On 10/13/2010 05:15 PM, David Rientjes wrote:
> ZONE_DMA is unnecessary for a large number of machines that do not
> require addressing in the lower 16MB of memory because they do not use
> ISA devices with 16-bit address registers (plus one page byte register).
> 
> This patch allows users to disable ZONE_DMA for x86 if they know they
> will not be using such devices with their kernel.
> 
> This prevents the VM from unnecessarily reserving a ratio of memory
> (defaulting to 1/256th of system capacity) with lowmem_reserve_ratio
> for such allocations when it will never be used.

This isn't true.

There are many, many devices other than ISA devices which need access to
a restricted-memory pool because of hardware DMA limitations.  This
seems like a really bad idea.

A much better idea would be to have a runtime option of setting the
reservation ratio, if recovering no more than 1/66th of a gigabyte
matters so much to you.

	-hpa
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