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

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Message-ID: <20161208114457.GA26794@node.shutemov.name>
Date:   Thu, 8 Dec 2016 14:44:57 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Jérémy Lefaure <jeremy.lefaure@....epita.fr>
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, Dec 08, 2016 at 01:50:11PM +0300, Kirill A. Shutemov 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?

Okay, I see the problem. It still doesn't make sense. Why would compiler
check for unused warnings before dropping unused code.

What about something like this, instead:

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 6f14de45b5ce..b538452a127e 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -180,7 +180,7 @@ static inline int split_huge_page(struct page *page)
 }
 static inline void deferred_split_huge_page(struct page *page) {}
 #define split_huge_pmd(__vma, __pmd, __address)        \
-       do { } while (0)
+       do { (void)__vma; } while (0)

 static inline void split_huge_pmd_address(struct vm_area_struct *vma,
                unsigned long address, bool freeze, struct page *page) {}
-- 
 Kirill A. Shutemov