linux-kernel - Re: [PATCH] mm/memory.c: Mark wp_huge_pmd() inline to prevent build failure

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Date:   Mon, 4 Dec 2017 08:54:44 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Linux MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] mm/memory.c: Mark wp_huge_pmd() inline to prevent build failure

On Sun, Dec 3, 2017 at 10:11 PM, Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
> With gcc 4.1.2:
>
>     mm/memory.o: In function `wp_huge_pmd':
>     memory.c:(.text+0x9b4): undefined reference to `do_huge_pmd_wp_page'
>
> Interestingly, wp_huge_pmd() is emitted in the assembler output, but
> never called.
>
> Apparently replacing the call to pmd_write() in __handle_mm_fault() by a
> call to the more complex pmd_access_permitted() reduced the ability of
> the compiler to remove unused code.

An alternative would be to start using #ifdefs and dummies for the
!CONFIG_TRANSPARENT_HUGEPAGE case, instead of relying on the varying degree
of ability of the various compiler versions to do dead code analysis and
elimination.

> Fix this by marking wp_huge_pmd() inline, like was done in commit
> 91a90140f9987101 ("mm/memory.c: mark create_huge_pmd() inline to prevent
> build failure") for a similar problem.
>
> Fixes: c7da82b894e9eef6 ("mm: replace pmd_write with pmd_access_permitted in fault + gup paths")
> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> ---
>  mm/memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 5eb3d2524bdc2823..f4d52847ca07a414 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3831,7 +3831,7 @@ static inline int create_huge_pmd(struct vm_fault *vmf)
>         return VM_FAULT_FALLBACK;
>  }
>
> -static int wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
> +static inline int wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
>  {
>         if (vma_is_anonymous(vmf->vma))
>                 return do_huge_pmd_wp_page(vmf, orig_pmd);

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds