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Date:   Sat, 25 Apr 2020 17:55:11 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Anshuman Khandual <anshuman.khandual@arm.com>
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Subject: Re: [PATCH 3/3] mm/hugetlb: Introduce
 HAVE_ARCH_CLEAR_HUGEPAGE_FLAGS
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On Tue, 14 Apr 2020 17:14:30 +0530 Anshuman Khandual <anshuman.khandual@arm.com> wrote:

> There are multiple similar definitions for arch_clear_hugepage_flags() on
> various platforms. This introduces HAVE_ARCH_CLEAR_HUGEPAGE_FLAGS for those
> platforms that need to define their own arch_clear_hugepage_flags() while
> also providing a generic fallback definition for others to use. This help
> reduce code duplication.
> 
> ...
>
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -544,6 +544,10 @@ static inline int is_hugepage_only_range(struct mm_struct *mm,
>  }
>  #endif
>  
> +#ifndef HAVE_ARCH_CLEAR_HUGEPAGE_FLAGS
> +static inline void arch_clear_hugepage_flags(struct page *page) { }
> +#endif
> +
>  #ifndef arch_make_huge_pte
>  static inline pte_t arch_make_huge_pte(pte_t entry, struct vm_area_struct *vma,
>  				       struct page *page, int writable)

This is the rather old-school way of doing it.  The Linus-suggested way is

#ifndef arch_clear_hugepage_flags
static inline void arch_clear_hugepage_flags(struct page *page)
{
}
#define arch_clear_hugepage_flags arch_clear_hugepage_flags
#endif

And the various arch headers do

static inline void arch_clear_hugepage_flags(struct page *page)
{
	<some implementation>
}
#define arch_clear_hugepage_flags arch_clear_hugepage_flags

It's a small difference - mainly to avoid adding two variables to the
overall namespace where one would do.