linux-kernel - Re: [PATCH] powerpc/mm: remove warning about ‘type’ being set

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Message-ID: <CA+7wUsy9j8Zfxs1dB-bR-dYrzWjpNRm=BZYqkvEH5c1UzHrZMA@mail.gmail.com>
Date:   Tue, 26 Jun 2018 08:25:44 +0200
From:   Mathieu Malaterre <malat@...ian.org>
To:     a.p.zijlstra@...llo.nl
Cc:     Christophe LEROY <christophe.leroy@....fr>,
        Michael Ellerman <mpe@...erman.id.au>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paul Mackerras <paulus@...ba.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] powerpc/mm: remove warning about ‘type’ being set

On Sat, Jun 23, 2018 at 7:12 PM christophe leroy
<christophe.leroy@....fr> wrote:
>
>
>
> Le 22/06/2018 à 21:27, Mathieu Malaterre a écrit :
> > ‘type’ is only used when CONFIG_DEBUG_HIGHMEM is set. So add a possibly
> > unused tag to variable. Remove warning treated as error with W=1:
> >
> >    arch/powerpc/mm/highmem.c:59:6: error: variable ‘type’ set but not used [-Werror=unused-but-set-variable]
>
> Is type neeeded at all when CONFIG_DEBUG_HIGHMEM is not set ?
>
> The call        type = kmap_atomic_idx();          seems useless when
> CONFIG_DEBUG_HIGHMEM isn't set. Couldn't we just most type definition
> and setting inside the CONFIG_DEBUG_HIGHMEM {} below ?
>
> Alternatively, maybe you could replace the #ifdef CONFIG_DEBUG_HIGHMEM
> by an    if (IS_ENABLED(CONFIG_DEBUG_HIGHMEM)) ?

I am not familiar with this code. But starring at other arch
implementations (eg. `arch/x86/mm/highmem_32.c`), it feels like
powerpc is skipping `pte_clear(&init_mm, vaddr, kmap_pte-idx);` unless
`CONFIG_DEBUG_HIGHMEM=y`. Could someone please confirm this is the
correct behavior ?

Or else I can rewrite the code a bit like `arch/arm/mm/highmem.c`
which skips `set_fixmap_pte(idx, __pte(0));` unless
`CONFIG_DEBUG_HIGHMEM=y`.

> Christophe
>
> >
> > Signed-off-by: Mathieu Malaterre <malat@...ian.org>
> > ---
> >   arch/powerpc/mm/highmem.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/mm/highmem.c b/arch/powerpc/mm/highmem.c
> > index 668e87d03f9e..82a0e37557a5 100644
> > --- a/arch/powerpc/mm/highmem.c
> > +++ b/arch/powerpc/mm/highmem.c
> > @@ -56,7 +56,7 @@ EXPORT_SYMBOL(kmap_atomic_prot);
> >   void __kunmap_atomic(void *kvaddr)
> >   {
> >       unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;
> > -     int type;
> > +     int type __maybe_unused;
> >
> >       if (vaddr < __fix_to_virt(FIX_KMAP_END)) {
> >               pagefault_enable();
> >
>
> ---
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