linux-kernel - Re: [PATCH] asm-generic: fix variable 'p4d' set but not used

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Date:   Tue, 6 Aug 2019 13:51:48 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Qian Cai <cai@....pw>, arnd@...db.de,
        kirill.shutemov@...ux.intel.com, mhocko@...e.com,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic: fix variable 'p4d' set but not used

On Tue, 6 Aug 2019 11:39:04 -0300 Jason Gunthorpe <jgg@...pe.ca> wrote:

> On Fri, Aug 02, 2019 at 03:41:22PM -0400, Qian Cai wrote:
> > GCC throws a warning on an arm64 system since the commit 9849a5697d3d
> > ("arch, mm: convert all architectures to use 5level-fixup.h"),
> > 
> > mm/kasan/init.c: In function 'kasan_free_p4d':
> > mm/kasan/init.c:344:9: warning: variable 'p4d' set but not used
> > [-Wunused-but-set-variable]
> >   p4d_t *p4d;
> >          ^~~
> > 
> > because p4d_none() in "5level-fixup.h" is compiled away while it is a
> > static inline function in "pgtable-nopud.h". However, if converted
> > p4d_none() to a static inline there, powerpc would be unhappy as it
> > reads those in assembler language in
> > "arch/powerpc/include/asm/book3s/64/pgtable.h",
> > 
> > ./include/asm-generic/5level-fixup.h: Assembler messages:
> > ./include/asm-generic/5level-fixup.h:20: Error: unrecognized opcode:
> > `static'
> > ./include/asm-generic/5level-fixup.h:21: Error: junk at end of line,
> > first unrecognized character is `{'
> > ./include/asm-generic/5level-fixup.h:22: Error: unrecognized opcode:
> > `return'
> > ./include/asm-generic/5level-fixup.h:23: Error: junk at end of line,
> > first unrecognized character is `}'
> > ./include/asm-generic/5level-fixup.h:25: Error: unrecognized opcode:
> > `static'
> > ./include/asm-generic/5level-fixup.h:26: Error: junk at end of line,
> > first unrecognized character is `{'
> > ./include/asm-generic/5level-fixup.h:27: Error: unrecognized opcode:
> > `return'
> > ./include/asm-generic/5level-fixup.h:28: Error: junk at end of line,
> > first unrecognized character is `}'
> > ./include/asm-generic/5level-fixup.h:30: Error: unrecognized opcode:
> > `static'
> > ./include/asm-generic/5level-fixup.h:31: Error: junk at end of line,
> > first unrecognized character is `{'
> > ./include/asm-generic/5level-fixup.h:32: Error: unrecognized opcode:
> > `return'
> > ./include/asm-generic/5level-fixup.h:33: Error: junk at end of line,
> > first unrecognized character is `}'
> > make[2]: *** [scripts/Makefile.build:375:
> > arch/powerpc/kvm/book3s_hv_rmhandlers.o] Error 1
> > 
> > Fix it by reference the variable in the macro instead.
> > 
> > Signed-off-by: Qian Cai <cai@....pw>
> >  include/asm-generic/5level-fixup.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/asm-generic/5level-fixup.h b/include/asm-generic/5level-fixup.h
> > index bb6cb347018c..2c3e14c924b6 100644
> > +++ b/include/asm-generic/5level-fixup.h
> > @@ -19,7 +19,7 @@
> >  
> >  #define p4d_alloc(mm, pgd, address)	(pgd)
> >  #define p4d_offset(pgd, start)		(pgd)
> > -#define p4d_none(p4d)			0
> > +#define p4d_none(p4d)			((void)p4d, 0)
> 
> Yuk, how about a static inline instead?

Yes.  With the appropriate `#ifndef __ASSEMBLY__' to avoid powerpc
build errors?