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

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Date:	Thu, 17 Nov 2011 15:57:50 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	David Daney <ddaney.cavm@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
	"ralf@...ux-mips.org" <ralf@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Daney <david.daney@...ium.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Robin Holt <holt@....com>
Subject: Re: [patch] hugetlb: remove dummy definitions of HPAGE_MASK and
 HPAGE_SIZE

On Thu, 17 Nov 2011, David Daney wrote:

> A counter argument would be:
> 
> There are hundreds of places in the kernel where dummy definitions are
> selected by !CONFIG_* so that we can do:
> 
>    if (test_something()) {
>       do_one_thing();
>    } else {
>       do_the_other_thing();
>    }
> 
> 
> Rather than:
> 
> #ifdef CONFIG_SOMETHING
>    if (test_something()) {
>       do_one_thing();
>    } else
> #else
>    {
>       do_the_other_thing();
>    }
> 
> 
> 
> We even do this all over the place with dummy definitions selected by
> CONFIG_HUGETLB_PAGE, What exactly makes HPAGE_MASK special and not the
> hundreds of other similar situations?
> 

Dummy functions that return 0 when a feature isn't enabled isn't the 
problem, that's very convenient.  Definitions of constants are a 
completely separate story because they can be easily used outside the 
required context and cause brekage.  What happens if you reference a 
variable in a function that is declarated in a different function?  Does 
the compiler put a BUG() in there and let you explode at runtime?  This is 
just absurd, every other arch has done the necessary declarations in their 
own header files and everything works fine because there's a clear config 
dependency on using HPAGE_*.  Adding dummy definitions to panic at runtime 
is irresponsibly to an extreme.
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