linux-kernel - Re: [PATCH] x86/vm86: fix compilation warning on a unused variable

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Message-ID: <20161208125110.7c17ab83@blatinox-arch>
Date:   Thu, 8 Dec 2016 12:51:10 -0500
From:   Jérémy Lefaure <jeremy.lefaure@....epita.fr>
To:     Borislav Petkov <bp@...e.de>
Cc:     x86@...nel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/vm86: fix compilation warning on a unused variable

On Thu, 8 Dec 2016 09:33:05 +0100
Borislav Petkov <bp@...e.de> wrote:

> On Wed, Dec 07, 2016 at 11:38:33PM -0500, Jérémy Lefaure wrote:
> > When CONFIG_TRANSPARENT_HUGEPAGE is disabled, split_huge_pmd is a no-op
> > stub. In such case, vma is unused and a compiler raises a warning:
> > 
> > arch/x86/kernel/vm86_32.c: In function ‘mark_screen_rdonly’:
> > arch/x86/kernel/vm86_32.c:180:26: warning: unused variable ‘vma’
> > [-Wunused-variable]
> >    struct vm_area_struct *vma = find_vma(mm, 0xA0000);
> >                              ^~~
> > Adding __maybe_unused in the vma declaration fixes this warning.
> > 
> > In addition, checking if CONFIG_TRANSPARENT_HUGEPAGE is enabled avoids
> > calling find_vma function for nothing.
> > 
> > Signed-off-by: Jérémy Lefaure <jeremy.lefaure@....epita.fr>
> > ---
> >  arch/x86/kernel/vm86_32.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
> > index 01f30e5..0813b76 100644
> > --- a/arch/x86/kernel/vm86_32.c
> > +++ b/arch/x86/kernel/vm86_32.c
> > @@ -176,8 +176,9 @@ static void mark_screen_rdonly(struct mm_struct *mm)
> >  		goto out;
> >  	pmd = pmd_offset(pud, 0xA0000);
> >  
> > -	if (pmd_trans_huge(*pmd)) {
> > -		struct vm_area_struct *vma = find_vma(mm, 0xA0000);
> > +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && pmd_trans_huge(*pmd)) {
> > +		struct vm_area_struct __maybe_unused *vma = find_vma(mm,
> > +								     0xA0000);  
> 
> So wouldn't the __maybe_unused alone without changing the if-condition
> fix the warning too?
> 

Yes it will. I did not see that pmd_trans_huge returns 0 if
CONFIG_TRANSPARENT_HUGEPAGE is disabled. So you're right, the
IS_ENABLED(...) in the condition is useless.

Thanks,
Jérémy