--- a/hotspot/src/os/posix/vm/os_posix.cpp	Wed Dec 12 12:07:32 2012 -0800
+++ b/hotspot/src/os/posix/vm/os_posix.cpp	Mon Dec 17 08:49:20 2012 +0100
@@ -93,6 +93,47 @@
   return;
 }
 
+// Multiple threads can race in this code, and can remap over each other with MAP_FIXED,
+// so on posix, unmap the section at the start and at the end of the chunk that we mapped
+// rather than unmapping and remapping the whole chunk to get requested alignment.
+char* os::reserve_memory_aligned(size_t size, size_t alignment) {
+  assert((alignment & (os::vm_allocation_granularity() - 1)) == 0,
+      "Alignment must be a multiple of allocation granularity (page size)");
+  assert((size & (alignment -1)) == 0, "size must be 'alignment' aligned");
+
+  size_t extra_size = size + alignment;
+  assert(extra_size >= size, "overflow, size is too large to allow alignment");
+
+  char* extra_base = os::reserve_memory(extra_size, NULL, alignment);
+
+  if (extra_base == NULL) {
+    return NULL;
+  }
+
+  // Do manual alignment
+  char* aligned_base = (char*) align_size_up((uintptr_t) extra_base, alignment);
+
+  // [  |                                       |  ]
+  // ^ extra_base
+  //    ^ extra_base + begin_offset == aligned_base
+  //     extra_base + begin_offset + size       ^
+  //                       extra_base + extra_size ^
+  // |<>| == begin_offset
+  //                              end_offset == |<>|
+  size_t begin_offset = aligned_base - extra_base;
+  size_t end_offset = (extra_base + extra_size) - (aligned_base + size);
+
+  if (begin_offset > 0) {
+      os::release_memory(extra_base, begin_offset);
+  }
+
+  if (end_offset > 0) {
+      os::release_memory(extra_base + begin_offset + size, end_offset);
+  }
+
+  return aligned_base;
+}
+
 void os::Posix::print_load_average(outputStream* st) {
   st->print("load average:");
   double loadavg[3];

--- a/hotspot/src/os/windows/vm/os_windows.cpp	Wed Dec 12 12:07:32 2012 -0800
+++ b/hotspot/src/os/windows/vm/os_windows.cpp	Mon Dec 17 08:49:20 2012 +0100
@@ -2895,6 +2895,36 @@
   }
 }
 
+// Multiple threads can race in this code but it's not possible to unmap small sections of
+// virtual space to get requested alignment, like posix-like os's.
+// Windows prevents multiple thread from remapping over each other so this loop is thread-safe.
+char* os::reserve_memory_aligned(size_t size, size_t alignment) {
+  assert((alignment & (os::vm_allocation_granularity() - 1)) == 0,
+      "Alignment must be a multiple of allocation granularity (page size)");
+  assert((size & (alignment -1)) == 0, "size must be 'alignment' aligned");
+
+  size_t extra_size = size + alignment;
+  assert(extra_size >= size, "overflow, size is too large to allow alignment");
+
+  char* aligned_base = NULL;
+
+  do {
+    char* extra_base = os::reserve_memory(extra_size, NULL, alignment);
+    if (extra_base == NULL) {
+      return NULL;
+    }
+    // Do manual alignment
+    aligned_base = (char*) align_size_up((uintptr_t) extra_base, alignment);
+
+    os::release_memory(extra_base, extra_size);
+
+    aligned_base = os::reserve_memory(size, aligned_base);
+
+  } while (aligned_base == NULL);
+
+  return aligned_base;
+}
+
 char* os::pd_reserve_memory(size_t bytes, char* addr, size_t alignment_hint) {
   assert((size_t)addr % os::vm_allocation_granularity() == 0,
          "reserve alignment");

--- a/hotspot/src/share/vm/runtime/os.hpp	Wed Dec 12 12:07:32 2012 -0800
+++ b/hotspot/src/share/vm/runtime/os.hpp	Mon Dec 17 08:49:20 2012 +0100
@@ -255,6 +255,7 @@
   static int    vm_allocation_granularity();
   static char*  reserve_memory(size_t bytes, char* addr = 0,
                                size_t alignment_hint = 0);
+  static char*  reserve_memory_aligned(size_t size, size_t alignment);
   static char*  attempt_reserve_memory_at(size_t bytes, char* addr);
   static void   split_reserved_memory(char *base, size_t size,
                                       size_t split, bool realloc);

--- a/hotspot/src/share/vm/runtime/virtualspace.cpp	Wed Dec 12 12:07:32 2012 -0800
+++ b/hotspot/src/share/vm/runtime/virtualspace.cpp	Mon Dec 17 08:49:20 2012 +0100
@@ -329,20 +329,9 @@
     if ((((size_t)base + noaccess_prefix) & (alignment - 1)) != 0) {
       // Base not aligned, retry
       if (!os::release_memory(base, size)) fatal("os::release_memory failed");
-      // Reserve size large enough to do manual alignment and
-      // increase size to a multiple of the desired alignment
+      // Make sure that size is aligned
       size = align_size_up(size, alignment);
-      size_t extra_size = size + alignment;
-      do {
-        char* extra_base = os::reserve_memory(extra_size, NULL, alignment);
-        if (extra_base == NULL) return;
-        // Do manual alignement
-        base = (char*) align_size_up((uintptr_t) extra_base, alignment);
-        assert(base >= extra_base, "just checking");
-        // Re-reserve the region at the aligned base address.
-        os::release_memory(extra_base, extra_size);
-        base = os::reserve_memory(size, base);
-      } while (base == NULL);
+      base = os::reserve_memory_aligned(size, alignment);
 
       if (requested_address != 0 &&
           failed_to_reserve_as_requested(base, requested_address, size, false)) {