linux-kernel - Re: [PATCH] mm: Mark create_huge_pmd() inline to prevent build failure

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Date:   Wed, 12 Jul 2017 17:29:00 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Linux MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Mark create_huge_pmd() inline to prevent build failure

On Tue, Jul 11, 2017 at 11:57 PM, Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
> With gcc 4.1.2:
>
>     mm/memory.o: In function `create_huge_pmd':
>     memory.c:(.text+0x93e): undefined reference to `do_huge_pmd_anonymous_page'
>
> Converting transparent_hugepage_enabled() from a macro to a static
> inline function reduced the ability of the compiler to remove unused
> code.
>
> Fix this by marking create_huge_pmd() inline.
>
> Fixes: 16981d763501c0e0 ("mm: improve readability of transparent_hugepage_enabled()")
> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> ---
> Interestingly, create_huge_pmd() is emitted in the assembler output, but
> never called.
> ---
>  mm/memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index cbb57194687e393a..0e517be91a89e162 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3591,7 +3591,7 @@ static int do_numa_page(struct vm_fault *vmf)
>         return 0;
>  }
>
> -static int create_huge_pmd(struct vm_fault *vmf)
> +static inline int create_huge_pmd(struct vm_fault *vmf)
>  {

This seems fragile, what if the kernel decides to ignore the inline
hint? If it must be inlined to avoid compile errors then it should be
__always_inline, right?

I also wonder if it's enough to just specify __always_inline to
transparent_hugepage_enabled(), i.e. in case the compiler is making an
uninlined copy of transparent_hugepage_enabled() in mm/memory.c.