Hi Kirill,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on next-20200729]
[also build test ERROR on v5.8-rc7]
[cannot apply to cgroup/for-next tip/timers/core net-next/master sparc-next/master net/master linus/master v5.8-rc7 v5.8-rc6 v5.8-rc5]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Kirill-Tkhai/proc-Introduce-proc-namespaces-directory-to-expose-namespaces-lineary/20200730-200346
base:    04b4571786305a76ad81757bbec78eb16a5de582
config: csky-defconfig (attached as .config)
compiler: csky-linux-gcc (GCC) 9.3.0
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
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=csky 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   csky-linux-ld: fs/proc/namespaces.o: in function `$d':
   namespaces.c:(.text+0x1d0): undefined reference to `userns_operations'
>> csky-linux-ld: namespaces.c:(.text+0x370): undefined reference to `userns_operations'

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org