Re: [linux-next:master 10286/12016] arch/arm64/mm/fixmap.c:199:10: warning: variable 'bm_pudp' set but not used

[Yujie Liu <yujie.liu@intel.com>]

On Mon, Apr 17, 2023 at 02:49:57PM +0100, Mark Rutland wrote:
> On Mon, Apr 17, 2023 at 08:55:17PM +0800, kernel test robot wrote:
> > Hi Mark,
> 
> Hi,
> 
> > FYI, the error/warning was bisected to this commit, please ignore it if it's irrelevant.
> 
> I took a look, and this warning existed in the prior commit. All that has
> changed is that it has moved from arch/arm64/mm/mmu.c to arch/arm64/mm/init.c.
> 
> This commit is not to blame, and there is no regression here.
> 
> The warning itself is bogus; more details on that below. Please ignore this.

Sorry for this false positive. We've configured the bot to filter out
the reports on this commit.


Best Regards,
Yujie

> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
> > head:   d3f2cd24819158bb70701c3549e586f9df9cee67
> > commit: b97547761b02cc95e0e6be827dc9ca9da8142761 [10286/12016] arm64: mm: move fixmap code to its own file
> > config: arm64-buildonly-randconfig-r004-20230416 (https://download.01.org/0day-ci/archive/20230417/202304172004.r3IPh5Ja-lkp@intel.com/config)
> > compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project 9638da200e00bd069e6dd63604e14cbafede9324)
> > reproduce (this is a W=1 build):
> >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> >         chmod +x ~/bin/make.cross
> >         # install arm64 cross compiling tool for clang build
> >         # apt-get install binutils-aarch64-linux-gnu
> >         # https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=b97547761b02cc95e0e6be827dc9ca9da8142761
> >         git remote add linux-next https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
> >         git fetch --no-tags linux-next master
> >         git checkout b97547761b02cc95e0e6be827dc9ca9da8142761
> >         # save the config file
> >         mkdir build_dir && cp config build_dir/.config
> >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=arm64 olddefconfig
> >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=arm64 SHELL=/bin/bash arch/arm64/mm/
> > 
> > If you fix the issue, kindly add following tag where applicable
> > | Reported-by: kernel test robot <lkp@intel.com>
> > | Link: https://lore.kernel.org/oe-kbuild-all/202304172004.r3IPh5Ja-lkp@intel.com/
> > 
> > All warnings (new ones prefixed by >>):
> > 
> > >> arch/arm64/mm/fixmap.c:199:10: warning: variable 'bm_pudp' set but not used [-Wunused-but-set-variable]
> >                    pud_t *bm_pudp;
> >                           ^
> >    1 warning generated.
> > 
> > 
> > vim +/bm_pudp +199 arch/arm64/mm/fixmap.c
> > 
> :>    182	
> >    183	/*
> >    184	 * Copy the fixmap region into a new pgdir.
> >    185	 */
> >    186	void __init fixmap_copy(pgd_t *pgdir)
> >    187	{
> >    188		if (!READ_ONCE(pgd_val(*pgd_offset_pgd(pgdir, FIXADDR_TOT_START)))) {
> >    189			/*
> >    190			 * The fixmap falls in a separate pgd to the kernel, and doesn't
> >    191			 * live in the carveout for the swapper_pg_dir. We can simply
> >    192			 * re-use the existing dir for the fixmap.
> >    193			 */
> >    194			set_pgd(pgd_offset_pgd(pgdir, FIXADDR_TOT_START),
> >    195				READ_ONCE(*pgd_offset_k(FIXADDR_TOT_START)));
> >    196		} else if (CONFIG_PGTABLE_LEVELS > 3) {
> >    197			pgd_t *bm_pgdp;
> >    198			p4d_t *bm_p4dp;
> >  > 199			pud_t *bm_pudp;
> 
> The full piece of code in question is:
> 
> |        } else if (CONFIG_PGTABLE_LEVELS > 3) {
> |                pgd_t *bm_pgdp;
> |                p4d_t *bm_p4dp;
> |                pud_t *bm_pudp;
>                         ^ defined
> 
> |                /*
> |                 * The fixmap shares its top level pgd entry with the kernel
> |                 * mapping. This can really only occur when we are running
> |                 * with 16k/4 levels, so we can simply reuse the pud level
> |                 * entry instead.
> |                 */
> |                BUG_ON(!IS_ENABLED(CONFIG_ARM64_16K_PAGES));
> |                bm_pgdp = pgd_offset_pgd(pgdp, FIXADDR_TOT_START);
> |                bm_p4dp = p4d_offset(bm_pgdp, FIXADDR_TOT_START);
> |                bm_pudp = pud_set_fixmap_offset(bm_p4dp, FIXADDR_TOT_START);
>                  ^ set
> 
> |                pud_populate(&init_mm, bm_pudp, lm_alias(bm_pmd));
>                                         ^ used
> 
> |                pud_clear_fixmap();
> |        } else {
> 
> What's happening here is that the config has CONFIG_PGTABLE_LEVELS=2, where the
> above block is unreachable, but the warning is being generated for trivially
> unreachable code.
> 
> With that config, we'll use include/asm-generic/pgtable-nopmd.h, where
> pud_populate() is defined as:
> 
> | #define pud_populate(mm, pmd, pte)          do { } while (0)
> 
> ... and hence it's complaing that the (unreachable) definition and
> initialization of bm_pudp is not used.
> 
> If I reconfigure the kernel with 4K pages and 48-bit VAs so that
> CONFIG_PGTABLE_LEVELS=4, the above warning isn't generated.
> 
> Thanks,
> Mark.
>