Hi Pavel,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on mmotm/master]
[also build test ERROR on v4.17-rc6]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Pavel-Tatashin/ksm-replace-jhash2-with-faster-hash/20180527-032512
base:   git://git.cmpxchg.org/linux-mmotm.git master
config: i386-randconfig-s0-201821 (attached as .config)
compiler: gcc-6 (Debian 6.4.0-9) 6.4.0 20171026
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   mm/ksm.c: In function 'fasthash':
>> mm/ksm.c:338:15: error: implicit declaration of function 'xxhash' [-Werror=implicit-function-declaration]
      return (u32)xxhash(input, length, 0);
                  ^~~~~~
   cc1: some warnings being treated as errors

vim +/xxhash +338 mm/ksm.c

   331	
   332	static u32 fasthash(const void *input, size_t length)
   333	{
   334		if (static_branch_likely(&ksm_use_crc32c))
   335			return crc32c(0, input, length);
   336	
   337		if (static_branch_likely(&ksm_use_xxhash))
 > 338			return (u32)xxhash(input, length, 0);
   339	
   340		/* Is done only once on the first call of fasthash() */
   341		fasthash_setup();
   342	
   343		/* Now, that we know the hash alg., calculate checksum for zero page */
   344		zero_checksum = fasthash(ZERO_PAGE(0), PAGE_SIZE);
   345	
   346		return fasthash(input, length);
   347	}
   348	

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