Negative values were returned by getFreeSwapSpaceSize() in our docker testing.
The reason is that current implementation doesn't consider the situation when memory.limit_in_bytes == memory.memsw.limit_in_bytes, which means do not use the swap space at all.

In src/jdk.management/unix/classes/com/sun/management/internal/OperatingSystemImpl.java, let's see
------------------------------------------------
 71 public long getFreeSwapSpaceSize() {
 72 if (containerMetrics != null) {
 73 long memSwapLimit = containerMetrics.getMemoryAndSwapLimit();
 74 long memLimit = containerMetrics.getMemoryLimit();
 75 if (memSwapLimit >= 0 && memLimit >= 0) {
 76 for (int attempt = 0; attempt < MAX_ATTEMPTS_NUMBER; attempt++) {
 77 long memSwapUsage = containerMetrics.getMemoryAndSwapUsage();
 78 long memUsage = containerMetrics.getMemoryUsage();
 79 if (memSwapUsage > 0 && memUsage > 0) {
 80 // We read "memory usage" and "memory and swap usage" not atomically,
 81 // and it's possible to get the negative value when subtracting these two.
 82 // If this happens just retry the loop for a few iterations.
 83 if ((memSwapUsage - memUsage) >= 0) {
 84 return memSwapLimit - memLimit - (memSwapUsage - memUsage);
 85 }
 86 }
 87 }
 88 }
 89 }
 90 return getFreeSwapSpaceSize0();
 91 }
------------------------------------------------
If memSwapLimit (@line 73) equals memLimit (@line 74), then getFreeSwapSpaceSize() may return a negative value @line 84.