linux-kernel - Re: [PATCH v3] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

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Message-ID: <3ffaa931-bc25-efff-173b-397824422842@redhat.com>
Date:   Thu, 31 Mar 2022 13:33:24 -0400
From:   Waiman Long <longman@...hat.com>
To:     kernel test robot <lkp@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kbuild-all@...ts.01.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Justin Forbes <jforbes@...hat.com>,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v3] mm/sparsemem: Fix 'mem_section' will never be NULL gcc
 12 warning

On 3/31/22 11:54, kernel test robot wrote:
> Hi Waiman,
>
> I love your patch! Perhaps something to improve:
>
> [auto build test WARNING on hnaz-mm/master]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Waiman-Long/mm-sparsemem-Fix-mem_section-will-never-be-NULL-gcc-12-warning/20220331-050049
> base:   https://github.com/hnaz/linux-mm master
> config: arm-randconfig-c024-20220330 (https://download.01.org/0day-ci/archive/20220331/202203312327.XGeCiD5T-lkp@intel.com/config)
> compiler: arm-linux-gnueabi-gcc (GCC) 11.2.0
> 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
>          # https://github.com/intel-lab-lkp/linux/commit/2098f1d78cde338e81b3ba596ea39f37824e496e
>          git remote add linux-review https://github.com/intel-lab-lkp/linux
>          git fetch --no-tags linux-review Waiman-Long/mm-sparsemem-Fix-mem_section-will-never-be-NULL-gcc-12-warning/20220331-050049
>          git checkout 2098f1d78cde338e81b3ba596ea39f37824e496e
>          # save the config file to linux build tree
>          mkdir build_dir
>          COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.2.0 make.cross O=build_dir ARCH=arm SHELL=/bin/bash security/keys/encrypted-keys/
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@...el.com>
>
> All warnings (new ones prefixed by >>):
>
>     In file included from include/linux/gfp.h:6,
>                      from include/linux/umh.h:4,
>                      from include/linux/kmod.h:9,
>                      from include/linux/module.h:17,
>                      from security/keys/encrypted-keys/encrypted.c:15:
>     security/keys/encrypted-keys/encrypted.c: In function 'derived_key_encrypt.constprop':
>>> include/linux/mmzone.h:1432:23: warning: array subscript 32 is outside array bounds of 'struct mem_section[32][1]' [-Warray-bounds]
>      1432 |         unsigned long map = section->section_mem_map;
>           |                       ^~~
>     include/linux/mmzone.h:1390:27: note: while referencing 'mem_section'
>      1390 | extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
>           |                           ^~~~~~~~~~~
>
>
> vim +1432 include/linux/mmzone.h
>
> 29751f6991e845 Andy Whitcroft 2005-06-23  1429
> 29751f6991e845 Andy Whitcroft 2005-06-23  1430  static inline struct page *__section_mem_map_addr(struct mem_section *section)
> 29751f6991e845 Andy Whitcroft 2005-06-23  1431  {
> 29751f6991e845 Andy Whitcroft 2005-06-23 @1432  	unsigned long map = section->section_mem_map;
> 29751f6991e845 Andy Whitcroft 2005-06-23  1433  	map &= SECTION_MAP_MASK;
> 29751f6991e845 Andy Whitcroft 2005-06-23  1434  	return (struct page *)map;
> 29751f6991e845 Andy Whitcroft 2005-06-23  1435  }
> 29751f6991e845 Andy Whitcroft 2005-06-23  1436
>
This warning happens with !CONFIG_SPARSEMEM_VMEMMAP && CONFIG_SPARSEMEM. 
In this case, __section_mem_map_addr() is used only by __pfn_to_page() 
and __page_to_pfn().  My patch only modifies __nr_to_section. In this 
particular case, it is removing The !mem_section[32] check. Perhaps it 
is somewhat equivalent to a (sec >= 32) check for gcc 11.2. However, 
this check will cause a warning in gcc 12. I will update the patch to 
add an explicit (sec >= NR_SECTION_ROOTS) then.

Thanks,
Longman