8330144: Revise os::free_memory()

Reviewed-by: stuefe, mbaesken

src/hotspot/os/aix/os_aix.cpp

@@ -1846,7 +1846,7 @@ bool os::remove_stack_guard_pages(char* addr, size_t size) {
 void os::pd_realign_memory(char *addr, size_t bytes, size_t alignment_hint) {
 }
 
-void os::pd_free_memory(char *addr, size_t bytes, size_t alignment_hint) {
+void os::pd_disclaim_memory(char *addr, size_t bytes) {
 }
 
 size_t os::pd_pretouch_memory(void* first, void* last, size_t page_size) {

src/hotspot/os/bsd/os_bsd.cpp

@@ -1684,7 +1684,7 @@ void os::pd_commit_memory_or_exit(char* addr, size_t size,
 void os::pd_realign_memory(char *addr, size_t bytes, size_t alignment_hint) {
 }
 
-void os::pd_free_memory(char *addr, size_t bytes, size_t alignment_hint) {
+void os::pd_disclaim_memory(char *addr, size_t bytes) {
   ::madvise(addr, bytes, MADV_DONTNEED);
 }
 

src/hotspot/os/linux/os_linux.cpp

@@ -3035,15 +3035,10 @@ void os::pd_realign_memory(char *addr, size_t bytes, size_t alignment_hint) {
   }
 }
 
-void os::pd_free_memory(char *addr, size_t bytes, size_t alignment_hint) {
-  // This method works by doing an mmap over an existing mmaping and effectively discarding
-  // the existing pages. However it won't work for SHM-based large pages that cannot be
-  // uncommitted at all. We don't do anything in this case to avoid creating a segment with
-  // small pages on top of the SHM segment. This method always works for small pages, so we
-  // allow that in any case.
-  if (alignment_hint <= os::vm_page_size() || can_commit_large_page_memory()) {
-    commit_memory(addr, bytes, alignment_hint, !ExecMem);
-  }
+// Hints to the OS that the memory is no longer needed and may be reclaimed by the OS when convenient.
+// The memory will be re-acquired on touch without needing explicit recommitting.
+void os::pd_disclaim_memory(char *addr, size_t bytes) {
+   ::madvise(addr, bytes, MADV_DONTNEED);
 }
 
 size_t os::pd_pretouch_memory(void* first, void* last, size_t page_size) {

src/hotspot/os/windows/os_windows.cpp

@@ -3893,7 +3893,7 @@ bool os::unguard_memory(char* addr, size_t bytes) {
 }
 
 void os::pd_realign_memory(char *addr, size_t bytes, size_t alignment_hint) { }
-void os::pd_free_memory(char *addr, size_t bytes, size_t alignment_hint) { }
+void os::pd_disclaim_memory(char *addr, size_t bytes) { }
 
 size_t os::pd_pretouch_memory(void* first, void* last, size_t page_size) {
   return page_size;

src/hotspot/share/gc/parallel/mutableNUMASpace.cpp

@@ -213,7 +213,7 @@ void MutableNUMASpace::bias_region(MemRegion mr, uint lgrp_id) {
     // Then we uncommit the pages in the range.
     // The alignment_hint argument must be less than or equal to the small page
     // size if not using large pages or else this function does nothing.
-    os::free_memory((char*)aligned_region.start(), aligned_region.byte_size(), os_align);
+    os::disclaim_memory((char*)aligned_region.start(), aligned_region.byte_size());
     // And make them local/first-touch biased.
     os::numa_make_local((char*)aligned_region.start(), aligned_region.byte_size(), checked_cast<int>(lgrp_id));
   }

src/hotspot/share/gc/parallel/mutableSpace.cpp

@@ -53,7 +53,7 @@ void MutableSpace::numa_setup_pages(MemRegion mr, size_t page_size, bool clear_s
       size_t size = pointer_delta(end, start, sizeof(char));
       if (clear_space) {
         // Prefer page reallocation to migration.
-        os::free_memory((char*)start, size, page_size);
+        os::disclaim_memory((char*)start, size);
       }
       os::numa_make_global((char*)start, size);
     }

src/hotspot/share/runtime/os.cpp

@@ -2271,8 +2271,8 @@ bool os::unmap_memory(char *addr, size_t bytes) {
   return result;
 }
 
-void os::free_memory(char *addr, size_t bytes, size_t alignment_hint) {
-  pd_free_memory(addr, bytes, alignment_hint);
+void os::disclaim_memory(char *addr, size_t bytes) {
+  pd_disclaim_memory(addr, bytes);
 }
 
 void os::realign_memory(char *addr, size_t bytes, size_t alignment_hint) {

src/hotspot/share/runtime/os.hpp

@@ -231,7 +231,7 @@ class os: AllStatic {
                            char *addr, size_t bytes, bool read_only = false,
                            bool allow_exec = false);
   static bool   pd_unmap_memory(char *addr, size_t bytes);
-  static void   pd_free_memory(char *addr, size_t bytes, size_t alignment_hint);
+  static void   pd_disclaim_memory(char *addr, size_t bytes);
   static void   pd_realign_memory(char *addr, size_t bytes, size_t alignment_hint);
 
   // Returns 0 if pretouch is done via platform dependent method, or otherwise
@@ -520,7 +520,7 @@ class os: AllStatic {
                            char *addr, size_t bytes, bool read_only = false,
                            bool allow_exec = false, MEMFLAGS flags = mtNone);
   static bool   unmap_memory(char *addr, size_t bytes);
-  static void   free_memory(char *addr, size_t bytes, size_t alignment_hint);
+  static void   disclaim_memory(char *addr, size_t bytes);
   static void   realign_memory(char *addr, size_t bytes, size_t alignment_hint);
 
   // NUMA-specific interface

test/hotspot/gtest/runtime/test_committed_virtualmemory.cpp

@@ -132,7 +132,7 @@ public:
     }
 
     // Cleanup
-    os::free_memory(base, size, page_sz);
+    os::disclaim_memory(base, size);
     VirtualMemoryTracker::remove_released_region((address)base, size);
 
     rmr = VirtualMemoryTracker::_reserved_regions->find(ReservedMemoryRegion((address)base, size));

test/hotspot/gtest/runtime/test_os.cpp

@@ -20,7 +20,6 @@
  * or visit www.oracle.com if you need additional information or have any
  * questions.
  */
-
 #include "precompiled.hpp"
 #include "memory/allocation.hpp"
 #include "memory/resourceArea.hpp"
@@ -977,3 +976,27 @@ TEST_VM(os, vm_min_address) {
 #endif
 }
 
+#if !defined(_WINDOWS) && !defined(_AIX)
+TEST_VM(os, free_without_uncommit) {
+  const size_t page_sz = os::vm_page_size();
+  const size_t pages = 64;
+  const size_t size = pages * page_sz;
+
+  char* base = os::reserve_memory(size, false, mtTest);
+  ASSERT_NE(base, (char*) nullptr);
+  ASSERT_TRUE(os::commit_memory(base, size, false));
+
+  for (size_t index = 0; index < pages; index++) {
+    base[index * page_sz] = 'a';
+  }
+
+  os::disclaim_memory(base, size);
+
+  // Ensure we can still use the memory without having to recommit.
+  for (size_t index = 0; index < pages; index++) {
+    base[index * page_sz] = 'a';
+  }
+
+  os::release_memory(base, size);
+}
+#endif