Hi Jia,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on linus/master]
[cannot apply to v5.3 next-20190917]
[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/Jia-He/fix-double-page-fault-on-arm64/20190918-220036
config: arm64-allnoconfig (attached as .config)
compiler: aarch64-linux-gcc (GCC) 7.4.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=7.4.0 make.cross ARCH=arm64 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <redacted>

All errors (new ones prefixed by >>):

   mm/memory.o: In function `wp_page_copy':

quoted

memory.c:(.text+0x8fc): undefined reference to `cpu_has_hw_af'

   memory.c:(.text+0x8fc): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `cpu_has_hw_af'

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