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Date: Fri, 8 May 2015 15:24:28 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Subject: Re: [PATCH V2 1/2] mm/thp: Split out pmd collpase flush into a
 seperate functions
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On Thu,  7 May 2015 12:53:27 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

> After this patch pmdp_* functions operate only on hugepage pte,
> and not on regular pmd_t values pointing to page table.
> 

The patch looks like a pretty safe no-op for non-powerpc?

> --- a/arch/powerpc/include/asm/pgtable-ppc64.h
> +++ b/arch/powerpc/include/asm/pgtable-ppc64.h
> @@ -576,6 +576,10 @@ static inline void pmdp_set_wrprotect(struct mm_struct *mm, unsigned long addr,
>  extern void pmdp_splitting_flush(struct vm_area_struct *vma,
>  				 unsigned long address, pmd_t *pmdp);
>  
> +#define __HAVE_ARCH_PMDP_COLLAPSE_FLUSH
> +extern pmd_t pmdp_collapse_flush(struct vm_area_struct *vma,
> +				 unsigned long address, pmd_t *pmdp);
> +

The fashionable way of doing this is

extern pmd_t pmdp_collapse_flush(struct vm_area_struct *vma,
				 unsigned long address, pmd_t *pmdp);
#define pmdp_collapse_flush pmdp_collapse_flush

then, elsewhere,

#ifndef pmdp_collapse_flush
static inline pmd_t pmdp_collapse_flush(...) {}
#define pmdp_collapse_flush pmdp_collapse_flush
#endif

It avoids introducing a second (ugly) symbol into the kernel.