src/hotspot/cpu/aarch64/gc/z/zGlobals_aarch64.cpp

*** 20,34 ****
--- 20,37 ----
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  #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.
  //
  // Address Space & Pointer Layout 1
  // --------------------------------
*** 133,145 ****
  //  |                                                  1000 = Finalizable  (Address view N/A)
  //  |
  //  * 63-48 Fixed (16-bits, always zero)
  //
  
  size_t ZPlatformAddressOffsetBits() {
!   const size_t min_address_offset_bits = 42; // 4TB
!   const size_t max_address_offset_bits = 44; // 16TB
    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);
  }
  
--- 136,181 ----
  //  |                                                  1000 = Finalizable  (Address view N/A)
  //  |
  //  * 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 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);
  }