Re: [PATCH] mm, THP: vmf_insert_pfn_pud depends on CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD

[Alexandre Ghiti]

On 12/01/2018 01:28, Andrew Morton wrote:
> On Thu, 11 Jan 2018 14:05:34 +0100 Alexandre Ghiti <aghiti@xxxxxxxxx> 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.