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SubjectRe: [patch] x86: allow ZONE_DMA to be configurable
On Thu, 14 Oct 2010, Andrew Morton 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.
> >
>
> I wonder how hard it would be to do this at runtime, probably with a
> boot parameter.
>

A "no_zone_dma" boot parameter wouldn't allow us to achieve the text or
data savings that we see from disabling all three options in question
here.

Hot-adding ZONE_DMA at runtime would be possible but there's no guarantee
that memory hasn't fully been used by the time you do it, so it disregards
lowmem_reserve_ratio unless you migrate everything, which has a dependency
on it being movable.

> I'd be a little concerned at the effects of this on page reclaim and
> the page allocator - it might expose weird pre-existing bugs or
> inefficiencies. But we can cross that bridge when we fall off it, I
> guess.
>

We've run with it for a couple years, we can even undefine __GFP_DMA to
find allocations that we compile into the kernel to ensure we don't have a
requirement for the zone. Perhaps only define the gfp flag when we have
CONFIG_ZONE_DMA and break users' builds until they disable options that
require it (or enable CONFIG_ZONE_DMA)?

It would be great if we could do "select ZONE_DMA" for all options that
require it, though.


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