MAXMEM_PFN
max_low_pfn = MAXMEM_PFN;
highmem_pages = max_pfn - MAXMEM_PFN;
if (highmem_pages + MAXMEM_PFN < max_pfn)
max_pfn = MAXMEM_PFN + highmem_pages;
if (highmem_pages + MAXMEM_PFN > max_pfn) {
pages_to_mb(max_pfn - MAXMEM_PFN),
max_pfn = MAXMEM_PFN;
if (max_pfn <= MAXMEM_PFN)