linux-kernel - Re: How to alloc highmem page below 4GB on i386?

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Message-ID: <20080705000259.3d74c5b6@mjolnir.drzeus.cx>
Date:	Sat, 5 Jul 2008 00:02:59 +0200
From:	Pierre Ossman <drzeus-list@...eus.cx>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: How to alloc highmem page below 4GB on i386?

On Fri, 4 Jul 2008 13:37:33 -0700
Arjan van de Ven <arjan@...radead.org> wrote:

> On Fri, 4 Jul 2008 22:23:23 +0200
> Pierre Ossman <drzeus-list@...eus.cx> wrote:
> > 
> > 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. :)
> 
> actually your impression is not correct. There's a difference between
> how many physical bits the bus has, and the logical data. Specifically,
> PCI (and PCIE etc) have something that's called "Dual Address Cycle",
> which is a pci bus transaction that sends the 64 bit address using 2
> cycles on the bus even if the buswidth is 32 bit (logically).
> 

Ah, I see. I have to admit to only have read the PCI spec briefly. :)

Still, the devices I'm poking have 32-bit fields, so the limitation is
still there for my case.

> > 
> > 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?
> 
> ok this is tricky and goes way deep into buddy allocator internals.
> On the highest level (2Mb chunks iirc, but it could be a bit or
> two bigger now) we allocate top down. But once we split such a top level
> chunk up, inside the chunk we allocate bottom up (so that the scatter
> gather IOs tend to group nicer). 
> In addition, the kernel will prefer allocating userspace/pagecache
> memory from highmem over lowmem, out of an effort to keep memory
> pressure in the lowmem zones lower.
> 

For the test I'm playing with, in does a second order allocation, which
I suppose has good odds of finding a suitable hole somewhere in the
upper GB.

Ah well, I suppose this highmem business will eventually blow over. ;)

Thanks
-- 
     -- Pierre Ossman

  Linux kernel, MMC maintainer        http://www.kernel.org
  rdesktop, core developer          http://www.rdesktop.org

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