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

[David Rientjes]

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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/