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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Dec 08, 2016 at 01:25:37PM -0500, Jérémy Lefaure wrote:
> On Thu, 8 Dec 2016 13:50:11 +0300
> "Kirill A. Shutemov" <kirill.shutemov@linux.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.

This doesn't make any sense, but this works too:

diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index e339717af265..63ddc598f5a9 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -160,6 +160,7 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)

 static void mark_screen_rdonly(struct mm_struct *mm)
 {
+       struct vm_area_struct *vma;
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
@@ -181,7 +182,7 @@ static void mark_screen_rdonly(struct mm_struct *mm)
        pmd = pmd_offset(pud, 0xA0000);

        if (pmd_trans_huge(*pmd)) {
-               struct vm_area_struct *vma = find_vma(mm, 0xA0000);
+               vma = find_vma(mm, 0xA0000);
                split_huge_pmd(vma, pmd, 0xA0000);
        }
        if (pmd_none_or_clear_bad(pmd))
-- 
 Kirill A. Shutemov