8272807: Permit use of memory concurrent with pretouch

Reviewed-by: shade, stuefe

src/hotspot/share/runtime/os.cpp

@@ -1745,13 +1745,31 @@ void os::print_memory_mappings(outputStream* st) {
   os::print_memory_mappings(nullptr, (size_t)-1, st);
 }
 
+// Pretouching must use a store, not just a load.  On many OSes loads from
+// fresh memory would be satisfied from a single mapped page containing all
+// zeros.  We need to store something to each page to get them backed by
+// their own memory, which is the effect we want here.  An atomic add of
+// zero is used instead of a simple store, allowing the memory to be used
+// while pretouch is in progress, rather than requiring users of the memory
+// to wait until the entire range has been touched.  This is technically
+// a UB data race, but doesn't cause any problems for us.
 void os::pretouch_memory(void* start, void* end, size_t page_size) {
-  for (volatile char *p = (char*)start; p < (char*)end; p += page_size) {
-    // Note: this must be a store, not a load. On many OSes loads from fresh
-    // memory would be satisfied from a single mapped page containing all zeros.
-    // We need to store something to each page to get them backed by their own
-    // memory, which is the effect we want here.
-    *p = 0;
+  assert(start <= end, "invalid range: " PTR_FORMAT " -> " PTR_FORMAT, p2i(start), p2i(end));
+  assert(is_power_of_2(page_size), "page size misaligned: %zu", page_size);
+  assert(page_size >= sizeof(int), "page size too small: %zu", page_size);
+  if (start < end) {
+    // We're doing concurrent-safe touch and memory state has page
+    // granularity, so we can touch anywhere in a page.  Touch at the
+    // beginning of each page to simplify iteration.
+    char* cur = static_cast<char*>(align_down(start, page_size));
+    void* last = align_down(static_cast<char*>(end) - 1, page_size);
+    assert(cur <= last, "invariant");
+    // Iterate from first page through last (inclusive), being careful to
+    // avoid overflow if the last page abuts the end of the address range.
+    for ( ; true; cur += page_size) {
+      Atomic::add(reinterpret_cast<int*>(cur), 0, memory_order_relaxed);
+      if (cur >= last) break;
+    }
   }
 }