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Subject: Re: 2G memory split
From: Dave Hansen <haveblue@us.ibm.com>
To: Linus Torvalds <torvalds@osdl.org>
Cc: Martin Bligh <mbligh@mbligh.org>, Jens Axboe <axboe@suse.de>,
       Byron Stanoszek <gandalf@winds.org>, Ingo Molnar <mingo@elte.hu>,
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Date: Tue, 10 Jan 2006 10:55:11 -0800
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On Tue, 2006-01-10 at 10:34 -0800, Linus Torvalds wrote:
> On Tue, 10 Jan 2006, Martin Bligh wrote:
> > 
> > The non-1GB-aligned ones need to be disbarred when PAE is on, I think.
> 
> Well, right now _all_ the non-3:1 cases need to be disbarred. I think we 
> depend on the kernel mapping only ever being the _one_ last entry in the 
> top-level page table, which is only true with the 3:1 mapping.

It actually "just works".  We have a 16GB machine that gets a lot of
filesystem activity and use a 2:2 split all the time.  Appended patch is
all that we need.

It was for other reasons at the time, but I think we fixed a bunch of
the multiple kernel mapping PMDs back in 2.5.  Some remnants of that
stuff are still around.

http://marc.theaimsgroup.com/?l=linux-kernel&m=104197008817507&w=2

diff -purN -X /home/dvhart/.diff.exclude /home/linux/views/linux-2.6.12/include/asm-i386/page.h 2.6.12-uptime/include/asm-i386/page.h
--- /home/linux/views/linux-2.6.12/include/asm-i386/page.h	2005-03-02 03:00:08.000000000 -0800
+++ 2.6.12-uptime/include/asm-i386/page.h	2005-07-27 11:53:40.000000000 -0700
@@ -122,9 +122,9 @@ extern int sysctl_legacy_va_layout;
 #endif /* __ASSEMBLY__ */
 
 #ifdef __ASSEMBLY__
-#define __PAGE_OFFSET		(0xC0000000)
+#define __PAGE_OFFSET		(0x80000000)
 #else
-#define __PAGE_OFFSET		(0xC0000000UL)
+#define __PAGE_OFFSET		(0x80000000UL)
 #endif
 
-- Dave