Subject: Re: [PATCH] mm, THP: vmf_insert_pfn_pud depends on
 CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org,
        kirill.shutemov@linux.intel.com, dan.j.williams@intel.com,
        zi.yan@cs.rutgers.edu, gregkh@linuxfoundation.org,
        n-horiguchi@ah.jp.nec.com, mark.rutland@arm.com,
        linux-kernel@vger.kernel.org
References: <1515660811-12293-1-git-send-email-aghiti@upmem.com>
 <20180111100620.GY1732@dhcp22.suse.cz>
 <71853228-0beb-1e69-df47-59fa1bc5bd2f@upmem.com>
 <20180111162825.4cdaba2a21d8f15b21c45c75@linux-foundation.org>
From: Alexandre Ghiti <aghiti@upmem.com>
Message-ID: <01df063e-8cfd-11fd-a335-1e4a26377f95@upmem.com>
Date: Fri, 12 Jan 2018 16:26:09 +0100
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Sender: linux-kernel-owner@vger.kernel.org

On 12/01/2018 01:28, Andrew Morton wrote:
> On Thu, 11 Jan 2018 14:05:34 +0100 Alexandre Ghiti <aghiti@upmem.com> wrote:
>
>> 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.
> Disagree.  We'd have to put an absolutely vast amount of complex and
> hard-to-maintain ifdefs in headers if we were to ensure that such
> errors were to be detected at compile time.
>
> Whereas if we defer the detection of the errors until link time (or
> depmod or modprobe time) then yes, a handful of people will detect
> their mistake a minute or three later but that's a small cost compared
> to permanently and badly messing up the header files.
Ok, thanks for your time and explanations.