linux-kernel - Re: [PATCH] Cast __page_to_pfn to unsigned long in CONFIG

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Date:	Mon, 19 Nov 2007 12:09:43 -0800
From:	Dave Hansen <haveblue@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, mbligh@...gle.com
Subject: Re: [PATCH] Cast __page_to_pfn to unsigned long in CONFIG_SPARSEMEM

On Mon, 2007-11-19 at 14:52 -0500, Mathieu Desnoyers wrote:
> * Dave Hansen (haveblue@...ibm.com) wrote:
> > On Mon, 2007-11-19 at 13:52 -0500, Mathieu Desnoyers wrote:
> > > > > 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. 
> > 
> > Casting the i386 one to be an unconditional 'unsigned long' shouldn't be
> > an issue.  We don't generally expect pfns to fit into ints anyway. 
> 
> So would this make sense ?
> 
> Cast __page_to_pfn to unsigned long in CONFIG_SPARSEMEM
> 
> Make sure the type returned by __page_to_pfn is always unsigned long. If we
> don't cast it explicitly, it can be int on i386, but long on x86_64. This is
> especially inelegant for printks.

The only thing I might suggest doing differently is actually using the
page_to_pfn() definition itself:

memory_model.h:#define page_to_pfn __page_to_pfn

The full inline function version should do this already, and we
shouldn't have any real direct __page_to_pfn() users anyway.    

-- Dave

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