Hi Michal,

[auto build test ERROR on v4.5-rc6]
[also build test ERROR on next-20160229]
[if your patch is applied to the wrong git tree, please drop us a note to help improving the system]

url:    https://github.com/0day-ci/linux/commits/Michal-Hocko/mm-proc-make-clear_refs-killable/20160301-020426
config: x86_64-lkp (attached as .config)
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All errors (new ones prefixed by >>):

   fs/proc/task_mmu.c: In function 'clear_refs_write':
>> fs/proc/task_mmu.c:1016:4: error: implicit declaration of function 'down_write_killable' [-Werror=implicit-function-declaration]
       if (down_write_killable(&mm->mmap_sem)) {
       ^
   cc1: some warnings being treated as errors

vim +/down_write_killable +1016 fs/proc/task_mmu.c

  1010				.test_walk = clear_refs_test_walk,
  1011				.mm = mm,
  1012				.private = &cp,
  1013			};
  1014	
  1015			if (type == CLEAR_REFS_MM_HIWATER_RSS) {
> 1016				if (down_write_killable(&mm->mmap_sem)) {
  1017					count = -EINTR;
  1018					goto out_mm;
  1019				}

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