Report used memory as zero when total memory cannot be obtained

server/src/main/java/org/elasticsearch/monitor/os/OsStats.java

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.monitor.os;
 
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
@@ -225,6 +227,8 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
 
     public static class Mem implements Writeable, ToXContentFragment {
 
+        private static final Logger logger = LogManager.getLogger(Mem.class);
+
         private final long total;
         private final long free;
 
@@ -253,6 +257,16 @@ public ByteSizeValue getTotal() {
         }
 
         public ByteSizeValue getUsed() {
+            if (total == 0) {
+                // The work in https://github.com/elastic/elasticsearch/pull/42725 established that total memory
+                // can be reported as negative in some cases. In those cases, we force it to zero in which case
+                // we can no longer correctly report the used memory as (total-free) and should report it as zero.
+                //
+                // We intentionally check for (total == 0) rather than (total - free < 0) so as not to hide
+                // cases where (free > total) which would be a different bug.
+                logger.warn("cannot compute used memory when total memory is 0 and free memory is " + free);
+                return new ByteSizeValue(0);
+            }
             return new ByteSizeValue(total - free);
         }
 

server/src/test/java/org/elasticsearch/monitor/os/OsStatsTests.java

@@ -25,6 +25,8 @@
 
 import java.io.IOException;
 
+import static org.hamcrest.Matchers.equalTo;
+
 public class OsStatsTests extends ESTestCase {
 
     public void testSerialization() throws IOException {
@@ -81,4 +83,9 @@ public void testSerialization() throws IOException {
         }
     }
 
+    public void testGetUsedMemoryWithZeroTotal() {
+        OsStats.Mem mem = new OsStats.Mem(0, 1);
+        assertThat(mem.getUsed().getBytes(), equalTo(0L));
+    }
+
 }