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Subject: Re: [PATCH] powerpc/mm: add ZONE_NORMAL zone for 64 bit kernel
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Tabi Timur-B04825 <B04825@freescale.com>
Date: Tue, 24 Jul 2012 14:45:57 +1000
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On Tue, 2012-07-24 at 04:04 +0000, Tabi Timur-B04825 wrote:
> Benjamin Herrenschmidt wrote:
> > Sure but I don't want to create the zones in the first place (and thus
> > introduce the added pressure on the memory management) on machines that
> > don't need it.
> 
> One thing that does confuse me -- by default, we don't create a 
> ZONE_NORMAL.  We only create a ZONE_DMA.  Why is that?  Shouldn't it be 
> the other way around?

Because ZONE_NORMAL allocations can be serviced from the ZONE_DMA while
the other way isn't possible.

Especially in the old days, there were quite a few cases of drivers
and/or subsystems who were a bit heavy handed at using ZONE_DMA, so not
having one would essentially make them not work at all.

Cheers,
Ben.