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

[David Rientjes]

On Wed, 13 Oct 2010, H. Peter Anvin 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.
> 

You can already set the lowmem_reserve_ratio via 
/proc/sys/vm/lowmem_reserve_ratio.

We've run without ZONE_DMA for a couple years because we don't have such 
hardware limitations and would appreciate the ability to disable it with a 
config option rather than hacking the kernel to get it to compile.  I'm 
sure Casey would as well as you can see in the thread "A question about 
ZONE_DMA".

I don't see the harm in being able to conveniently disable the zone if you 
know what you're doing.
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