VMEMMAP_SIZE
#define VMEMMAP_START (VMEMMAP_END - VMEMMAP_SIZE)
min(PTRS_PER_PGD * PTRS_PER_PUD * PTRS_PER_PMD * PTRS_PER_PTE * PAGE_SIZE, (1UL << cpu_vabits)) - PMD_SIZE - VMEMMAP_SIZE - KFENCE_AREA_SIZE)
min(PTRS_PER_PGD * PTRS_PER_PUD * PTRS_PER_PMD * PTRS_PER_PTE * PAGE_SIZE, (1UL << cpu_vabits) / 2) - PMD_SIZE - VMEMMAP_SIZE - KFENCE_AREA_SIZE)
#define VMEMMAP_END ((unsigned long)vmemmap + VMEMMAP_SIZE - 1)
#ifndef VMEMMAP_SIZE
#define VMEMMAP_START (VMALLOC_START - VMEMMAP_SIZE)