linux-kernel - Re: [PATCH 9/9] mm: Remove ifdef condition in include/linux/mm.h

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Date:	Fri, 7 Feb 2014 15:27:12 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rashika Kheria <rashika.kheria@...il.com>,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Michel Lespinasse <walken@...gle.com>, linux-mm@...ck.org
Subject: Re: [PATCH 9/9] mm: Remove ifdef condition in include/linux/mm.h

On Fri, Feb 07, 2014 at 03:09:09PM -0800, David Rientjes wrote:
> On Fri, 7 Feb 2014, Andrew Morton wrote:
> 
> > > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > > > > index 1cedd00..5f8348f 100644
> > > > > > --- a/include/linux/mm.h
> > > > > > +++ b/include/linux/mm.h
> > > > > > @@ -1589,10 +1589,8 @@ static inline int __early_pfn_to_nid(unsigned long pfn)
> > > > > >  #else
> > > > > >  /* please see mm/page_alloc.c */
> > > > > >  extern int __meminit early_pfn_to_nid(unsigned long pfn);
> > > > > > -#ifdef CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID
> > > > > >  /* there is a per-arch backend function. */
> > > > > >  extern int __meminit __early_pfn_to_nid(unsigned long pfn);
> > > > > > -#endif /* CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID */
> > > > > >  #endif
> > > > > >  
> > > > > >  extern void set_dma_reserve(unsigned long new_dma_reserve);
> > > > > 
> > > > > Wouldn't it be better to just declare the __early_pfn_to_nid() in 
> > > > > mm/page_alloc.c to be static?
> > > > 
> > > > Won't that break the ability to override that function in
> > > > architecture-specific code (as arch/ia64/mm/numa.c does)?
> > > > 
> > > 
> > > Why?  CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID should define where this function 
> > > is defined so ia64 should have it set and the definition which I'm 
> > > suggesting be static is only compiled when this is undefined in 
> > > mm/page_alloc.c.  I'm not sure why we'd want to be messing with the 
> > > declaration?
> > 
> > __early_pfn_to_nid() must be global if it is implemented in arch/. 
> > 
> 
> Why??  If CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID then, yes, we need it to be 
> global.  Otherwise it's perfectly fine just being static in file scope.  
> This causes the compilation unit to break when you compile it, not wait 
> until vmlinux and find undefined references.
> 
> I see no reason it can't be done like this in mm/page_alloc.c:
> 
> 	#ifdef CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID
> 	extern int __meminit __early_pfn_to_nid(unsigned long pfn);

No, a .c file should not have an extern declaration in it.  This should
live in an appropriate header file, to be included in both page_alloc.c
and any arch file that defines an overriding function.

> Both of these options look much better than
> 
> 	include/linux/mm.h:
> 
> 	#if !defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP) && \
> 	    !defined(CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID)
> 	static inline int __early_pfn_to_nid(unsigned long pfn)
> 	{
> 	        return 0;
> 	}
> 	#else
> 	/* please see mm/page_alloc.c */
> 	extern int __meminit early_pfn_to_nid(unsigned long pfn);
> 	#ifdef CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID
> 	/* there is a per-arch backend function. */
> 	extern int __meminit __early_pfn_to_nid(unsigned long pfn);
> 	#endif /* CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID */
> 	#endif
> 
> where all this confusion is originating from.

The proposal is to first simplify those ifdefs by eliminating the inner
one in the #else; I agree with Andrew that we ought to go ahead and take
that step given the patch at hand, and then figure out if there's an
additional simplification possible.

- Josh Triplett
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