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

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Date:	Fri, 7 Feb 2014 17:25:16 -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 05:02:02PM -0800, David Rientjes wrote:
> On Fri, 7 Feb 2014, Josh Triplett wrote:
> 
> > > 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.
> > 
> 
> Ok, so you have religious beliefs about extern being used in files ending 
> in .c and don't mind the 2900 occurrences of it in the kernel tree and 
> desire 14 line obfuscation in header files with comments to what is being 
> defined in .c files such as "please see mm/page_alloc.c" as mm.h has.  

I (and many others) have very specific technical objections to not
having the same prototype seen by both the definition and use of a
function: that helps keep the prototype in sync with the definition(s),
helps keep all definitions in sync if there are multiple such
definitions, and eliminates -Wmissing-prototype warnings (which in
addition to those benefits also help detect functions that should be
made static or eliminated).

And as mentioned before, those 14 lines can be significantly simplified;
Rashika's patch already does one such simplification.

Those 2900 occurrences should go away as well, and Rashika's previous
patches have already fixed many of them.

> > > 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.
> > 
> 
> If additional simplification is possible?  Yeah, it's __weak which is 
> designed for this purpose.

No objections here if someone wants to write that patch.

- Josh Triplett
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