CONFIG_SPARSEMEM (not configurable)

modulename: discontig.ko

and/or

sparse.ko

configname: CONFIG_SPARSEMEM

Linux Kernel Configuration
└─>SPARSEMEM
In linux kernel since version 2.6.13 (release Date: 2005-08-28)  
source code:
mm/sparse.c

is selected by
CONFIG_MEMORY_HOTPLUG