linux-kernel - Re: [PATCH] mm: Fix BUILD_BUG assertion in pmdp_clear_flush

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Message-ID: <alpine.DEB.2.00.1112191125320.19949@chino.kir.corp.google.com>
Date:	Mon, 19 Dec 2011 11:29:36 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	David Daney <ddaney.cavm@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Paul Mundt <lethal@...ux-sh.org>, linux-kernel@...r.kernel.org,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH] mm: Fix BUILD_BUG assertion in
 pmdp_clear_flush_young()

On Mon, 19 Dec 2011, David Daney wrote:

> From: David Daney <david.daney@...ium.com>
> 
> It has been reported that when building for sh:
> 
>     mm/pgtable-generic.c: In function 'pmdp_clear_flush_young':
>     mm/pgtable-generic.c:76: error: call to '__build_bug_failed' declared with attribute error: BUILD_BUG failed
> 
> Adding some ugle #ifdeffery will probably fix this issue.
> 

It won't, CONFIG_HUGETLB_PAGE has nothing at all to do with this.

> Signed-off-by: David Daney <david.daney@...ium.com>
> ---
> 
> This is completely untested, but may be the best fix for this.  Linus
> and others have expressed a dislike for using BUG() to detect what
> should be dead code paths at runtime, but it seems to be the state of
> the art here.
> 

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.

>  mm/pgtable-generic.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index eb663fb..73784c8 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -70,7 +70,7 @@ int pmdp_clear_flush_young(struct vm_area_struct *vma,
>  			   unsigned long address, pmd_t *pmdp)
>  {
>  	int young;
> -#ifndef CONFIG_TRANSPARENT_HUGEPAGE
> +#if !defined(CONFIG_HUGETLB_PAGE) || !defined(CONFIG_TRANSPARENT_HUGEPAGE)
>  	BUG();
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
--
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