--- old/src/share/vm/logging/logStream.cpp	2017-06-21 17:45:59.011284600 +0200
+++ new/src/share/vm/logging/logStream.cpp	2017-06-21 17:45:58.112609800 +0200
@@ -41,7 +41,7 @@
 
 void LogStream::LineBuffer::ensure_cap(size_t atleast) {
   assert(_cap >= sizeof(_smallbuf), "sanity");
-  if (_cap < atleast) { 
+  if (_cap < atleast) {
     const size_t newcap = MAX2(_cap * 2, atleast * 2);
     char* const newbuf = (char*) os::malloc(newcap, mtLogging);
     if (_pos > 0) { // preserve old content
@@ -58,20 +58,20 @@
 
 void LogStream::LineBuffer::append(const char* s, size_t len) {
   assert(_buf[_pos] == '\0', "sanity");
-  const size_t remaining = _cap - _pos;
-  ensure_cap(remaining + len + 1);
+  ensure_cap(_pos + len + 1);
+  assert(_cap >= _pos + len + 1, "sanity");
   memcpy(_buf + _pos, s, len);
   _pos += len;
   _buf[_pos] = '\0';
 }
-    
+
 void LogStream::LineBuffer::reset() {
   _pos = 0;
   _buf[_pos] = '\0';
 }
 
 void LogStream::write(const char* s, size_t len) {
-  
+
   if (len > 0 && s[len - 1] == '\n') {
     _current_line.append(s, len - 1); // omit the newline.
     _log_handle.print("%s", _current_line.ptr());
--- old/src/share/vm/logging/logStream.hpp	2017-06-21 17:46:05.206783000 +0200
+++ new/src/share/vm/logging/logStream.hpp	2017-06-21 17:46:04.525735300 +0200
@@ -32,6 +32,7 @@
 
 
 class LogStream : public outputStream {
+  friend class LogStreamTest_TestLineBufferAllocation_test_vm_Test; // see test/native/logging/test_logStream.cpp
 
   // Helper class, maintains the line buffer. For small line lengths,
   // we avoid malloc and use a fixed sized member char array. If LogStream
@@ -49,14 +50,14 @@
     const char* ptr() const { return _buf; }
     void append(const char* s, size_t len);
     void reset();
-  }; 
+  };
   LineBuffer  _current_line;
   LogTargetHandle _log_handle;
 
   // Prevent operator new for LogStream.
   static void* operator new (size_t);
   static void* operator new[] (size_t);
-  
+
 public:
   // Constructor to support creation from a LogTarget instance.
   //
--- old/test/native/logging/test_logStream.cpp	2017-06-21 17:46:10.583527700 +0200
+++ new/test/native/logging/test_logStream.cpp	2017-06-21 17:46:09.857743600 +0200
@@ -66,3 +66,25 @@
   LogStream ls(Log(gc)::debug());
   verify_stream(&ls);
 }
+
+TEST_VM_F(LogStreamTest, TestLineBufferAllocation) {
+  const int max_line_len = 1024;
+  char* const test_string = (char*) os::malloc(max_line_len, mtLogging);
+  memset(test_string, 'A', max_line_len);
+  for (int interval = 1; interval < max_line_len; interval++) {
+    LogStream ls(Log(logging)::info());
+    int written = 0;
+    while (written < max_line_len) {
+      const int to_write = MIN2(interval, max_line_len - written);
+      ls.write(test_string, interval);
+      written += interval;
+      const char* const line_buffer = ls._current_line.ptr();
+      for (int i = 0; i < written; i++) {
+        ASSERT_TRUE(line_buffer[i] == 'A');
+      }
+      ASSERT_TRUE(line_buffer[written] == '\0');
+    }
+    ls.cr(); // I do not expect printout, nor do I care. Just call cr() to flush and avoid assert in ~LogStream().
+  }
+}
+