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Date: Fri, 10 Jan 2014 00:13:44 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Michal Hocko <mhocko@suse.cz>
Cc: David Rientjes <rientjes@google.com>, linux-kernel@vger.kernel.org,
        Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org,
        Dave Hansen <dave.hansen@intel.com>
Subject: Re: [RFC] mm: show message when updating min_free_kbytes in thp
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On Fri, 10 Jan 2014 09:05:04 +0100 Michal Hocko <mhocko@suse.cz> wrote:

> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -100,6 +100,7 @@ static struct khugepaged_scan khugepaged_scan = {
> > >  	.mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
> > >  };
> > >  
> > > +extern int user_min_free_kbytes;
> > >  
> > 
> > We don't add extern declarations to .c files.  How many other examples of 
> > this can you find in mm/?
> 
> I have suggested this because general visibility is not needed.

It's best to use a common declaration which is seen by the definition
site and all references, so everyone agrees on the variable's type. 
Otherwise we could have "long foo;" in one file and "extern char foo;"
in another and the compiler won't tell us.  I think the linker could
tell us, but it doesn't, afaik.  Perhaps there's an option...

> But if
> you think that it should then include/linux/mm.h sounds like a proper
> place.

mm/internal.h might suit.