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SubjectRe: [PATCH] mm: Fix BUILD_BUG assertion in pmdp_clear_flush_young()
On Mon, 19 Dec 2011, David Daney wrote:

> > The problem here is that HPAGE_PMD_MASK is being referenced without
> > CONFIG_TRANSPARENT_HUGEPAGE. pmdp_clear_flush_young() is only applicable
> > with that config option enabled. So rather than trying to minimize lines
> > captured under an #ifdef by placing the BUG() directly in the function for
> > asm-generic, you'll need to wrap pmd_clear_flush_young() with
> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE and define its equivalent for
> > CONFIG_TRANSPARENT_HUGEPAGE=n that breaks the build if not enabled.
> >
> That does sound like a better option.
> I still think having the BUILD_BUG defined as we do in Andrew Morton's tree is
> the correct approach, and I don't want to see that reverted because of this
> ugliness.

Agreed, the problem here is that we have existing code, such as
pmdp_clear_flush_young() that is calling BUG() at runtime for
non-supported configurations. So when BUILD_BUG_ON() for HPAGE_PMD_MASK
and friends was introduced, it detects improper use that needs to be fixed
and you've found one of those cases.

> Really this is hackery that should be cleaned up as you suggest, instead of
> being papered over with this BUG() thing. I can't test SH things, otherwise I
> might look at it. The SH maintainers should probably take a look instead.

It's not only about sh, this will affect any architecture that uses the
generic version of pmdp_clear_flush_young(), which is everything except

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