--- old/src/hotspot/cpu/aarch64/gc/z/zGlobals_aarch64.cpp	2020-08-31 10:13:25.428201265 +0200
+++ new/src/hotspot/cpu/aarch64/gc/z/zGlobals_aarch64.cpp	2020-08-31 10:13:25.316201316 +0200
@@ -22,11 +22,14 @@
  */
 
 #include "precompiled.hpp"
+#include "gc/shared/gcLogPrecious.hpp"
 #include "gc/z/zGlobals.hpp"
 #include "runtime/globals.hpp"
 #include "utilities/globalDefinitions.hpp"
 #include "utilities/powerOfTwo.hpp"
 
+#include <sys/mman.h>
+
 //
 // The heap can have three different layouts, depending on the max heap size.
 //
@@ -135,9 +138,42 @@
 //  * 63-48 Fixed (16-bits, always zero)
 //
 
+static size_t probe_valid_max_address_bit() {
+  const size_t page_size = (size_t) sysconf(_SC_PAGE_SIZE);
+  size_t max_address_bits = 0;
+  // TODO: Iterate from high to low addresses and stop at the first valid one, should be faster.
+  for (size_t i = 1; i <= sizeof(uintptr_t) * CHAR_BIT; ++i) {
+    const uintptr_t base_addr = ((uintptr_t) 1U) << i;
+    if (msync((void*)base_addr, page_size, MS_ASYNC) == 0) {
+      // msync suceeded, the address is valid, and maybe even already mapped.
+      max_address_bits = i;
+      continue;
+    }
+    if (errno != ENOMEM) {
+      // Some error occured. This should never happen, but msync
+      // has some undefined behavior, hence ignore this bit.
+      continue;
+    }
+    // Since msync failed with ENOMEM, the page might not be mapped.
+    // Try to map it, to see if the address is valid.
+    void* result_addr = mmap((void*) base_addr, page_size, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    if ((uintptr_t) result_addr == base_addr) {
+      // address is valid
+      max_address_bits = i;
+    }
+    if (result_addr != MAP_FAILED) {
+      munmap(result_addr, page_size);
+    }
+  }
+  max_address_bits += 1;
+  log_info_p(gc, init)("Probing address space for the number of valid bits: %zu", max_address_bits);
+  return max_address_bits;
+}
+
 size_t ZPlatformAddressOffsetBits() {
-  const size_t min_address_offset_bits = 42; // 4TB
-  const size_t max_address_offset_bits = 44; // 16TB
+  const static size_t ZAddressBits = probe_valid_max_address_bit();
+  const size_t max_address_offset_bits = ZAddressBits - 3;
+  const size_t min_address_offset_bits = max_address_offset_bits - 2;
   const size_t address_offset = round_up_power_of_2(MaxHeapSize * ZVirtualToPhysicalRatio);
   const size_t address_offset_bits = log2_intptr(address_offset);
   return clamp(address_offset_bits, min_address_offset_bits, max_address_offset_bits);