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To: suparna@in.ibm.com
Cc: linux-kernel@vger.kernel.org, lkcd-devel@lists.sourceforge.net,
       fastboot@osdl.org, anton@samba.org
Subject: Re: [Fastboot] Re: Kexec on 2.5.59 problems ?
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From: ebiederm@xmission.com (Eric W. Biederman)
Date: 18 Feb 2003 08:06:42 -0700
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Suparna Bhattacharya <suparna@in.ibm.com> writes:

> Here's the explanation from Anton about why using init_mm is
> a problem on ppc64.

Thanks.

> Hi Suparna,
> 
> On ppc64 we have many 2^41B (2 TB) regions:
> 
> USER
> KERNEL
> VMALLOC
> IO
> 
> Why 2TB? Well our three level linux pagetables can map 2TB. The kernel has
> no pagetables, so we only need three sets of pagetables. As usual each
> user task has its own set of pagetables. So that leaves vmalloc and IO.
> 
> For IO we create our own pgd, ioremap_pgd and for vmalloc we use init_mm.
> Why not? Its not being used anywhere else... except for kexec.
> 
> So init_mm covers the region of:
> 
> 0xD000000000000000 to 0xD000000000000000+2^41
> 
> And what kexec wants is a page under 4GB :)

In this case it definitely wants something identity mapped, which would
mean in the first 2TB region.  On x86 the limit is 4GB because I only have
32bit pointers.  On a 64bit arch that limit should go away.

> Thats why we created another mm.

That makes sense.    I guess it boils down to the fact that init_mm
is special cased in a number of places and using it I am likely to get
me into trouble...

You would not happen to have code that creates a separate mm so I can
be lazy would you?

Eric
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