linux-kernel - Re: [PATCH 41/75] hexagon: Add pmd

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Date:   Sun, 6 Feb 2022 22:05:35 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 41/75] hexagon: Add pmd_pfn()

On Sun, Feb 06, 2022 at 11:33:40PM +0200, Mike Rapoport wrote:
> On Sun, Feb 06, 2022 at 08:46:45PM +0000, Matthew Wilcox wrote:
> > On Sun, Feb 06, 2022 at 08:13:11PM +0200, Mike Rapoport wrote:
> > > On Fri, Feb 04, 2022 at 07:58:18PM +0000, Matthew Wilcox (Oracle) wrote:
> > > > I need to use this function in common code, so define it for hexagon.
> > > > 
> > > > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > > > ---
> > > >  arch/hexagon/include/asm/pgtable.h | 3 ++-
> > > 
> > > Why hexagon out of all architectures?
> > > What about m68k, nios2, nds32 etc?
> 
> > Presumably they don't support CONFIG_TRANSPARENT_HUGEPAGE.
> > This code isn't compiled when THP are disabled; at least I haven't
> > had a buildbot complaint for any other architectures.
> 
> m68k defconfig fails:
> 
>   CC      mm/page_vma_mapped.o
> mm/page_vma_mapped.c: In function 'page_vma_mapped_walk':
> mm/page_vma_mapped.c:219:20: error: implicit declaration of function 'pmd_pfn'; did you mean 'pmd_off'? [-Werror=implicit-function-declaration]
>   219 |     if (!check_pmd(pmd_pfn(pmde), pvmw))
>       |                    ^~~~~~~
>       |                    pmd_off

Ok, so this whole thing is protected by "if (pmd_trans_huge(pmd))"
which is defined to be 0 if !THP.  But obviously, gcc is trying to parse
it.  So I think we need something like:

#ifndef CONFIG_TRANSPARENT_HUGEPAGE
static inline unsigned long pmd_pfn(pmd_t pmd)
{
        BUG();
        return 0UL;
}
#endif

but that can have its own problem if architectures define pmd_pfn()
when they don't have CONFIG_TRANSPARENT_HUGEPAGE.  Which of course, they
can do.