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Date:	Fri, 4 Jul 2008 22:23:23 +0200
From:	Pierre Ossman <>
To:	Arjan van de Ven <>
Cc:	LKML <>,
Subject: Re: How to alloc highmem page below 4GB on i386?

On Fri, 4 Jul 2008 11:12:24 -0700
Arjan van de Ven <> wrote:

> On Fri, 4 Jul 2008 19:58:00 +0200
> Pierre Ossman <> wrote:
> > On Mon, 30 Jun 2008 20:03:23 +0200
> > Pierre Ossman <> wrote:
> > 
> > > Simple question. How do I allocate a page from highmem, that's still
> > > within 32 bits? x86_64 has the DMA32 zone, but i386 has just
> > > HIGHMEM. As most devices can't DMA above 32 bit, I have 3 GB of
> > > memory that's not getting decent usage (or results in needless
> > > bouncing). What to do?
> > > 
> > > I tried just enabling CONFIG_DMA32 for i386, but there is some guard
> > > against too many memory zones. I'm assuming this is there for a good
> > > reason?
> > > 
> > 
> > Anyone?
> > 
> well... the assumption sort of is that all high-perf devices are 64 bit
> capable. For the rest... well you get what you get. There's IOMMU's in
> modern systems from Intel (and soon AMD) that help you avoid the bounce
> if you really care. 

I was under the impression that the PCI bus was utterly incapable of
any larger address than 32 bits? But perhaps you only consider PCIE
stuff high-perf. :)

> The second assumption sort of is that you don't have 'too much' above
> 4Gb; once you're over 16Gb or so people assume you will run the 64 bit
> kernel instead...

Unfortunately some proprietary crud keeps migration somewhat annoying.
And in my case it's a 4 GB system, where 1 GB gets mapped up to make
room for devices, so it's not that uncommon.

The strange thing is that I keep getting pages from > 4GB all the time,
even on a loaded system. I would have expected mostly getting pages
below that limit as that's where most of the memory is. Do you have any
insight into which areas tend to fill up first?

     -- Pierre Ossman

  Linux kernel, MMC maintainer
  rdesktop, core developer

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