linux-kernel - Re: [PATCH] mm, THP: vmf_insert_pfn_pud depends on CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE

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Date:   Thu, 11 Jan 2018 14:05:34 +0100
From:   Alexandre Ghiti <aghiti@...em.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        kirill.shutemov@...ux.intel.com, dan.j.williams@...el.com,
        zi.yan@...rutgers.edu, gregkh@...uxfoundation.org,
        n-horiguchi@...jp.nec.com, willy@...ux.intel.com,
        mark.rutland@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, THP: vmf_insert_pfn_pud depends on
 CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD

On 11/01/2018 11:06, Michal Hocko wrote:
> On Thu 11-01-18 09:53:31, Alexandre Ghiti wrote:
>> The only definition of vmf_insert_pfn_pud depends on
>> CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD being defined. Then its declaration in
>> include/linux/huge_mm.h should have the same restriction so that we do
>> not expose this function if CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD is
>> not defined.
> Why is this a problem? Compiler should simply throw away any
> declarations which are not used?
It is not a big problem but surrounding the declaration with the #ifdef 
makes the compilation of external modules fail with an "error: implicit 
declaration of function vmf_insert_pfn_pud" if 
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD is not defined. I think it is 
cleaner than generating a .ko which would not load anyway.

>
>> Signed-off-by: Alexandre Ghiti <aghiti@...em.com>
>> ---
>>   include/linux/huge_mm.h | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index a8a1262..11794f6a 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -48,8 +48,10 @@ extern int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>>   			int prot_numa);
>>   int vmf_insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>>   			pmd_t *pmd, pfn_t pfn, bool write);
>> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
>>   int vmf_insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
>>   			pud_t *pud, pfn_t pfn, bool write);
>> +#endif
>>   enum transparent_hugepage_flag {
>>   	TRANSPARENT_HUGEPAGE_FLAG,
>>   	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
>> -- 
>> 2.1.4
>>