Hi Michal,

[auto build test WARNING on next-20160229]
[also build test WARNING on v4.5-rc6]
[cannot apply to drm/drm-next drm-intel/for-linux-next v4.5-rc6 v4.5-rc5 v4.5-rc4]
[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/change-mmap_sem-taken-for-write-killable/20160229-213258
config: i386-randconfig-x003-201609 (attached as .config)
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All warnings (new ones prefixed by >>):

   In file included from include/asm-generic/bug.h:4:0,
                    from arch/x86/include/asm/bug.h:35,
                    from include/linux/bug.h:4,
                    from include/linux/mmdebug.h:4,
                    from include/linux/mm.h:8,
                    from fs/proc/task_mmu.c:1:
   fs/proc/task_mmu.c: In function 'clear_refs_write':
   fs/proc/task_mmu.c:1030:8: error: implicit declaration of function 'down_write_killable' [-Werror=implicit-function-declaration]
       if (down_write_killable(&mm->mmap_sem)) {
           ^
   include/linux/compiler.h:151:30: note: in definition of macro '__trace_if'
     if (__builtin_constant_p(!!(cond)) ? !!(cond) :   \
                                 ^
>> fs/proc/task_mmu.c:1030:4: note: in expansion of macro 'if'
       if (down_write_killable(&mm->mmap_sem)) {
       ^
   cc1: some warnings being treated as errors

vim +/if +1030 fs/proc/task_mmu.c

  1014		task = get_proc_task(file_inode(file));
  1015		if (!task)
  1016			return -ESRCH;
  1017		mm = get_task_mm(task);
  1018		if (mm) {
  1019			struct clear_refs_private cp = {
  1020				.type = type,
  1021			};
  1022			struct mm_walk clear_refs_walk = {
  1023				.pmd_entry = clear_refs_pte_range,
  1024				.test_walk = clear_refs_test_walk,
  1025				.mm = mm,
  1026				.private = &cp,
  1027			};
  1028	
  1029			if (type == CLEAR_REFS_MM_HIWATER_RSS) {
> 1030				if (down_write_killable(&mm->mmap_sem)) {
  1031					put_task_struct(task);
  1032					return -EINTR;
  1033				}
  1034	
  1035				/*
  1036				 * Writing 5 to /proc/pid/clear_refs resets the peak
  1037				 * resident set size to this mm's current rss value.
  1038				 */

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