RFR: 8242480: Negative value may be returned by getFreeSwapSpaceSize() in the docker

[Severin Gehwolf]

Hi Jie,

On Fri, 2020-04-10 at 01:49 +0000, jiefu(傅杰) wrote:
> Hi all,
>  
> JBS:    https://bugs.openjdk.java.net/browse/JDK-8242480
> Webrev: http://cr.openjdk.java.net/~jiefu/8242480/webrev.00/
>  
> 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.
>  
> It would be better to fix it.
> Could you please review it and give me some advice?

Would this be reproducible via a test? There is
test/hotspot/jtreg/containers/docker/TestMemoryAwareness.java which
contains testOperatingSystemMXBeanAwareness() tests.

It would be good to capture this in a test somehow.

Thanks,
Severin