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

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Date:   Thu, 8 Dec 2016 13:25:37 -0500
From:   Jérémy Lefaure <jeremy.lefaure@....epita.fr>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     x86@...nel.org, Borislav Petkov <bp@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/vm86: fix compilation warning on a unused variable

On Thu, 8 Dec 2016 13:50:11 +0300
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> 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.  
> 
> Hm. pmd_trans_huge() is zero if CONFIG_TRANSPARENT_HUGEPAGE is not set.
> Compiler should get rid of whole block of code under the 'if'.
> 
> Could you share your kernel config which triggers the warning?
> And what compiler do you use?
> 

After a `make allnoconfig`, I enable "Legacy VM86 support" and nothing
else. I tested with 2 compilers, gcc 4.9.2 (on debian jessie) and gcc
6.2.1 (on archlinux).

Actually, the compiler does not raise warnings on complete build (`make
mrproper`, configuration and `make`) but only on partial build (`make
arch/x86/kernel/vm86_32.o` or `touch arch/x86/kernel/vm86_32.c &&
make`). So maybe it is a compiler issue ?

The solution you propose in your other email (adding "(void)__vma;" in
the no-op split_huge_pmd) seems to fix the warnings on partial build.

Thanks,
Jérémy