linux-kernel - Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and HPAGE

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Date:	Mon, 21 Nov 2011 15:53:31 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Daney <ddaney.cavm@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	linux-mips@...ux-mips.org, ralf@...ux-mips.org,
	linux-kernel@...r.kernel.org, David Daney <david.daney@...ium.com>,
	linux-arch@...r.kernel.org, Robin Holt <holt@....com>
Subject: Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and
 HPAGE_SIZE

On Mon, 21 Nov 2011 15:47:32 -0800
David Daney <ddaney.cavm@...il.com> wrote:

> Just to expand on this lovely topic...
> 
> On 11/21/2011 02:43 PM, Linus Torvalds wrote:
> > On Mon, Nov 21, 2011 at 2:23 PM, David Daney<ddaney.cavm@...il.com>  wrote:
> >>
> >> This whole comment strikes me as somewhat dishonest, as at the time David
> >> Rientjes wrote it, he knew that there were dependencies on these symbols in
> >> the linux-next tree.
> >>
> >> Now we can add these:
> >> +#define HPAGE_SHIFT    ({ BUG(); 0; })
> >> +#define HPAGE_SIZE     ({ BUG(); 0; })
> >> +#define HPAGE_MASK     ({ BUG(); 0; })
> >
> > Hell no.
> >
> > We don't do run-time BUG() things. No way, no how.
> >
> 
> These symbols are on dead code paths, so they are eliminated by the 
> compiler's Dead Code Elimination (DCE) optimizations, and the BUG() code 
> never gets emitted to the final executable.
> 
> I agree that it is not the best thing to do, but given the current state 
> of the art in build bug macros, it is the best we could have done.
> 
> What I think we need instead, and for which I will send a patch soon, is 
> something like this:
> 
> extern void you_are_screwed() __attribute__ ((error("BUILD_BUG_ON_USED 
> failed")));
> #define BUILD_BUG_ON_USED() you_are_screwed()
> #define HPAGE_SHIFT    ({ BUILD_BUG_ON_USED(); 0; })
> 
> This allows us to use the symbols in straight line C code without a ton 
> of ugly #ifdefery, but give us build time error checking.

The way we usually handle that is to emit a call to a
this_function_does_not_exist(), so it fails at link time.

I guess we should do that to all those follow_hugetlb_page() and
friends.

gcc has had issues at times where it incorrectly emits references to
this_function_does_not_exist(), but that hasn't happened in a couple of
years as far as I know.

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