linux-kernel - Re: [RFC 5/7] LTTng instrumentation mm

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Date:	Mon, 19 Nov 2007 13:52:58 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Dave Hansen <haveblue@...ibm.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, mbligh@...gle.com
Subject: Re: [RFC 5/7] LTTng instrumentation mm

* Dave Hansen (haveblue@...ibm.com) wrote:
> On Fri, 2007-11-16 at 09:47 -0500, Mathieu Desnoyers wrote:
> > * Dave Hansen (haveblue@...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'
> 

Well, using page_to_pfn turns out to be ugly in markers (and in
printks) then. Depending on the architecture, it will result in either
an unsigned long (x86_64) or an unsigned int (i386), which corresponds
to %lu or %u and will print a warning if we don't cast it explicitly.

Mathieu


> -- Dave
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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