--- old/src/share/vm/logging/logStream.cpp	2017-07-17 12:20:59.488598600 +0200
+++ new/src/share/vm/logging/logStream.cpp	2017-07-17 12:20:58.825393600 +0200
@@ -46,14 +46,20 @@
 void LogStream::LineBuffer::try_ensure_cap(size_t atleast) {
   assert(_cap >= sizeof(_smallbuf), "sanity");
   if (_cap < atleast) {
+    // Cap out at a reasonable max to prevent runaway leaks.
     const size_t reasonable_max = 1 * M;
-    size_t newcap = align_size_up(atleast + 64, 64);
     assert(_cap <= reasonable_max, "sanity");
-    // Cap out at a reasonable max to prevent runaway leaks.
+    if (_cap == reasonable_max) {
+      return;
+    }
+
+    size_t newcap = align_up(atleast + 64, 64);
     if (newcap > reasonable_max) {
+      log_info(logging)("Suspiciously long log line: \"%.100s%s",
+              _buf, (_pos >= 100 ? "..." : ""));
       newcap = reasonable_max;
     }
-    
+
     char* const newbuf = (char*) os::malloc(newcap, mtLogging);
     if (newbuf == NULL) { // OOM. Leave object unchanged.
       return;
--- old/test/native/logging/test_logStream.cpp	2017-07-17 12:21:05.999165700 +0200
+++ new/test/native/logging/test_logStream.cpp	2017-07-17 12:21:05.360176600 +0200
@@ -97,4 +97,6 @@
   }
   const char* const line_buffer = ls._current_line.ptr();
   ASSERT_TRUE(strlen(line_buffer) == 1*M - 1);
+  // reset to prevent assert for unflushed content
+  ls._current_line.reset();
 }