linux-kernel - Re: [PATCH 16 of 66] special pmd_trans

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Date:	Thu, 18 Nov 2010 12:51:12 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 16 of 66] special pmd_trans_* functions

On Wed, Nov 03, 2010 at 04:27:51PM +0100, Andrea Arcangeli wrote:
> From: Andrea Arcangeli <aarcange@...hat.com>
> 
> These returns 0 at compile time when the config option is disabled, to allow
> gcc to eliminate the transparent hugepage function calls at compile time
> without additional #ifdefs (only the export of those functions have to be
> visible to gcc but they won't be required at link time and huge_memory.o can be
> not built at all).
> 
> _PAGE_BIT_UNUSED1 is never used for pmd, only on pte.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> Acked-by: Rik van Riel <riel@...hat.com>
> ---
> 
> diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
> --- a/arch/x86/include/asm/pgtable_64.h
> +++ b/arch/x86/include/asm/pgtable_64.h
> @@ -168,6 +168,19 @@ extern void cleanup_highmap(void);
>  #define	kc_offset_to_vaddr(o) ((o) | ~__VIRTUAL_MASK)
>  
>  #define __HAVE_ARCH_PTE_SAME
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static inline int pmd_trans_splitting(pmd_t pmd)
> +{
> +	return pmd_val(pmd) & _PAGE_SPLITTING;
> +}
> +
> +static inline int pmd_trans_huge(pmd_t pmd)
> +{
> +	return pmd_val(pmd) & _PAGE_PSE;
> +}
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
>  #endif /* !__ASSEMBLY__ */
>  
>  #endif /* _ASM_X86_PGTABLE_64_H */
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -22,6 +22,7 @@
>  #define _PAGE_BIT_PAT_LARGE	12	/* On 2MB or 1GB pages */
>  #define _PAGE_BIT_SPECIAL	_PAGE_BIT_UNUSED1
>  #define _PAGE_BIT_CPA_TEST	_PAGE_BIT_UNUSED1
> +#define _PAGE_BIT_SPLITTING	_PAGE_BIT_UNUSED1 /* only valid on a PSE pmd */
>  #define _PAGE_BIT_NX           63       /* No execute: only valid after cpuid check */
>  
>  /* If _PAGE_BIT_PRESENT is clear, we use these: */
> @@ -45,6 +46,7 @@
>  #define _PAGE_PAT_LARGE (_AT(pteval_t, 1) << _PAGE_BIT_PAT_LARGE)
>  #define _PAGE_SPECIAL	(_AT(pteval_t, 1) << _PAGE_BIT_SPECIAL)
>  #define _PAGE_CPA_TEST	(_AT(pteval_t, 1) << _PAGE_BIT_CPA_TEST)
> +#define _PAGE_SPLITTING	(_AT(pteval_t, 1) << _PAGE_BIT_SPLITTING)
>  #define __HAVE_ARCH_PTE_SPECIAL
>  
>  #ifdef CONFIG_KMEMCHECK
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -348,6 +348,11 @@ extern void untrack_pfn_vma(struct vm_ar
>  				unsigned long size);
>  #endif
>  
> +#ifndef CONFIG_TRANSPARENT_HUGEPAGE
> +#define pmd_trans_huge(pmd) 0
> +#define pmd_trans_splitting(pmd) 0
> +#endif
> +

Usually it is insisted upon that this looks like

static inline int pmd_trans_huge(pmd) {
	return 0;
}

I understand it's to avoid any possibility of side-effets though to have type
checking and I am 99% certain the compiler still does the right thing. Still,
with no obvious side-effects here;

Acked-by: Mel Gorman <mel@....ul.ie>

>  #endif /* !__ASSEMBLY__ */
>  
>  #endif /* _ASM_GENERIC_PGTABLE_H */
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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