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

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Date:   Thu, 13 Jul 2017 09:04:42 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Dan Williams <dan.j.williams@...el.com>
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

Hi Dan,

On Thu, Jul 13, 2017 at 2:29 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> 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?

With gcc-4, "inline" is already #define'd to
#define inline inline           __attribute__((always_inline,unused)) notrace

> 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.

Hence the answer is no.

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