PRESERVE_BITS
bits = xa_load(&physxa->phys_bits, pfn_high / PRESERVE_BITS);
clear_bit(pfn_high % PRESERVE_BITS, bits->preserve);
bits = xa_load_or_alloc(&physxa->phys_bits, pfn_high / PRESERVE_BITS);
set_bit(pfn_high % PRESERVE_BITS, bits->preserve);
elm->phys_start = (phys * PRESERVE_BITS)
for_each_set_bit(bit, bitmap->preserve, PRESERVE_BITS) {
DECLARE_BITMAP(preserve, PRESERVE_BITS);