mem_section_usage_size
extern size_t mem_section_usage_size(void);
mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
sparse_usagebuf = (void *)sparse_usagebuf + mem_section_usage_size();
size = mem_section_usage_size() * map_count;
usage = kzalloc(mem_section_usage_size(), GFP_KERNEL);