On Fri, 2018-07-06 at 17:56 -0400, Rik van Riel wrote:
> The mm_struct always contains a cpumask bitmap, regardless of
> CONFIG_CPUMASK_OFFSTACK. That means the first step can be to
> simplify things, and simply have one bitmask at the end of the
> mm_struct for the mm_cpumask.

Otherwise virgin master.today grumbles.

  CC      kernel/bounds.s
  UPD     include/generated/timeconst.h
  UPD     include/generated/bounds.h
  CC      arch/x86/kernel/asm-offsets.s
  UPD     include/generated/asm-offsets.h
  CALL    scripts/checksyscalls.sh
  CHK     include/generated/compile.h
  HOSTCC  usr/gen_init_cpio
  UPD     include/generated/compile.h
  CC      init/main.o
In file included from ./include/linux/cpumask.h:12:0,
                 from ./arch/x86/include/asm/cpumask.h:5,
                 from ./arch/x86/include/asm/msr.h:11,
                 from ./arch/x86/include/asm/processor.h:21,
                 from ./arch/x86/include/asm/cpufeature.h:5,
                 from ./arch/x86/include/asm/thread_info.h:53,
                 from ./include/linux/thread_info.h:38,
                 from ./arch/x86/include/asm/preempt.h:7,
                 from ./include/linux/preempt.h:81,
                 from ./include/linux/spinlock.h:51,
                 from ./include/linux/seqlock.h:36,
                 from ./include/linux/time.h:6,
                 from ./include/linux/stat.h:19,
                 from ./include/linux/module.h:10,
                 from init/main.c:16:
In function ‘bitmap_zero’,
    inlined from ‘cpumask_clear’ at ./include/linux/cpumask.h:378:2,
    inlined from ‘mm_init_cpumask’ at ./include/linux/mm_types.h:504:2,
    inlined from ‘start_kernel’ at init/main.c:560:2:
./include/linux/bitmap.h:208:3: warning: ‘memset’ writing 64 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]
   memset(dst, 0, len);
   ^~~~~~~~~~~~~~~~~~~