linux-kernel - Re: [PATCH] thp: define HPAGE_PMD_* constans as BUILD

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Date:	Mon, 17 Jun 2013 15:33:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] thp: define HPAGE_PMD_* constans as BUILD_BUG() if !THP

On Tue, 18 Jun 2013 01:27:03 +0300 (EEST) "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:

> > >  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> > > +#define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
> > > +#define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
> > > +#define HPAGE_PMD_SIZE ({ BUILD_BUG(); 0; })
> > >  
> > 
> > We've done this sort of thing before and it blew up.  We do want to be
> > able to use things like HPAGE_PMD_foo in global-var initialisers and
> > definitions, but the problem is that BUILD_BUG() can't be used outside
> > functions.
> 
> I don't see how it's a blocker. For global variables, we will have to use
> #ifdefs, but the approach is still useful for in-function code.

OK.  Current mainline uses BUILD_BUG() here, so I guess the change
won't break anything.  Yet.

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