CONFIG_HIGHMEM is not available for thedefaultarchitecture x86.
Result is shown for architecture xtensa

High memory support

modulename: mm/highmem.ko

and/or

highmem.ko

configname: CONFIG_HIGHMEM

Linux/Xtensa Kernel Configuration
└─>Processor type and features
└─>High memory support
In linux kernel since version 2.6.12