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Subject: Re: [RFC 5/7] LTTng instrumentation mm
From: Dave Hansen <haveblue@us.ibm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, mbligh@google.com
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Date: Mon, 19 Nov 2007 10:07:05 -0800
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On Fri, 2007-11-16 at 09:47 -0500, Mathieu Desnoyers wrote:
> * Dave Hansen (haveblue@us.ibm.com) wrote:
> > For most (all?) architectures, the PFN and the virtual address in the
> > kernel's linear are interchangeable with pretty trivial arithmetic.  All
> > pages have a pfn, but not all have a virtual address.  Thus, I suggested
> > using the pfn.  What kind of virtual addresses are you talking about?
> > 
> 
> Hrm, in asm-generic/memory_model.h, we have various versions of
> __page_to_pfn. Normally they all cast the result to (unsigned long),
> except for :
> 
> 
> #elif defined(CONFIG_SPARSEMEM_VMEMMAP)
> 
> /* memmap is virtually contigious.  */
> #define __pfn_to_page(pfn)      (vmemmap + (pfn))
> #define __page_to_pfn(page)     ((page) - vmemmap)
> 
> So I guess the result is a pointer ? Should this be expected ?

Nope.  'pointer - pointer' is an integer.  Just solve this equation for
integer:

	'pointer + integer = pointer'

-- Dave