quoted

- I don't understand why this is necessary.  memory_is_poisoned_16()
  already handles unaligned addresses?

- If it's needed on ARM then presumably it will be needed on other
  architectures, so CONFIG_ARM is insufficiently general.

- If the present memory_is_poisoned_16() indeed doesn't work on ARM,
  it would be better to generalize/fix it in some fashion rather than
  creating a new variant of the function.

Yes, I think it will be better to fix the current function rather then
have 2 slightly different copies with ifdef's.
Will something along these lines work for arm? 16-byte accesses are
not too common, so it should not be a performance problem. And
probably modern compilers can turn 2 1-byte checks into a 2-byte check
where safe (x86).

static __always_inline bool memory_is_poisoned_16(unsigned long addr)
{
       u8 *shadow_addr = (u8 *)kasan_mem_to_shadow((void *)addr);

       if (shadow_addr[0] || shadow_addr[1])
               return true;
       /* Unaligned 16-bytes access maps into 3 shadow bytes. */
       if (unlikely(!IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
               return memory_is_poisoned_1(addr + 15);
       return false;
}

Thanks for Andrew Morton and Dmitry Vyukov's review. 
If the parameter addr=0xc0000008, now in function:
static __always_inline bool memory_is_poisoned_16(unsigned long addr)
{
 ---     //shadow_addr = (u16 *)(KASAN_OFFSET+0x18000001(=0xc0000008>>3)) is not 
 ---     // unsigned by 2 bytes.
        u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr); 

        /* Unaligned 16-bytes access maps into 3 shadow bytes. */
        if (unlikely(!IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
                return *shadow_addr || memory_is_poisoned_1(addr + 15);
----      //here is going to be error on arm, specially when kernel has not finished yet.
----      //Because the unsigned accessing cause DataAbort Exception which is not
----      //initialized when kernel is starting. 
        return *shadow_addr;
}

I also think it is better to fix this problem.