Do not report negative values for swap sizes (elastic#57317)

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

@@ -46,8 +46,9 @@
  * The {@link OsProbe} class retrieves information about the physical and swap size of the machine
  * memory, as well as the system load average and cpu load.
  *
- * In some exceptional cases, it's possible the underlying native method used by
- * {@link #getFreePhysicalMemorySize()} and {@link #getTotalPhysicalMemorySize()} can return a
+ * In some exceptional cases, it's possible the underlying native methods used by
+ * {@link #getFreePhysicalMemorySize()}, {@link #getTotalPhysicalMemorySize()},
+ * {@link #getFreeSwapSpaceSize()}, and {@link #getTotalSwapSpaceSize()} can return a
  * negative value. Because of this, we prevent those methods from returning negative values,
  * returning 0 instead.
  *
@@ -127,12 +128,19 @@ public long getTotalPhysicalMemorySize() {
      */
     public long getFreeSwapSpaceSize() {
         if (getFreeSwapSpaceSize == null) {
-            return -1;
+            logger.warn("getFreeSwapSpaceSize is not available");
+            return 0;
         }
         try {
-            return (long) getFreeSwapSpaceSize.invoke(osMxBean);
+            final long mem = (long) getFreeSwapSpaceSize.invoke(osMxBean);
+            if (mem < 0) {
+                logger.warn("OS reported a negative free swap space size [{}]", mem);
+                return 0;
+            }
+            return mem;
         } catch (Exception e) {
-            return -1;
+            logger.warn("exception retrieving free swap space size", e);
+            return 0;
         }
     }
 
@@ -141,12 +149,19 @@ public long getFreeSwapSpaceSize() {
      */
     public long getTotalSwapSpaceSize() {
         if (getTotalSwapSpaceSize == null) {
-            return -1;
+            logger.warn("getTotalSwapSpaceSize is not available");
+            return 0;
         }
         try {
-            return (long) getTotalSwapSpaceSize.invoke(osMxBean);
+            final long mem = (long) getTotalSwapSpaceSize.invoke(osMxBean);
+            if (mem < 0) {
+                logger.warn("OS reported a negative total swap space size [{}]", mem);
+                return 0;
+            }
+            return mem;
         } catch (Exception e) {
-            return -1;
+            logger.warn("exception retrieving total swap space size", e);
+            return 0;
         }
     }
 

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

@@ -183,17 +183,23 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
 
     public static class Swap implements Writeable, ToXContentFragment {
 
+        private static final Logger logger = LogManager.getLogger(Swap.class);
+
         private final long total;
         private final long free;
 
         public Swap(long total, long free) {
+            assert total >= 0 : "expected total swap to be positive, got: " + total;
+            assert free >= 0 : "expected free swap to be positive, got: " + total;
             this.total = total;
             this.free = free;
         }
 
         public Swap(StreamInput in) throws IOException {
             this.total = in.readLong();
+            assert total >= 0 : "expected total swap to be positive, got: " + total;
             this.free = in.readLong();
+            assert free >= 0 : "expected free swap to be positive, got: " + total;
         }
 
         @Override
@@ -207,6 +213,17 @@ public ByteSizeValue getFree() {
         }
 
         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. Swap can similarly be reported as negative and in
+                // those cases, we force it to zero in which case we can no longer correctly report the used swap
+                // 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 swap when total swap is 0 and free swap is " + free);
+                return new ByteSizeValue(0);
+            }
             return new ByteSizeValue(total - free);
         }
 

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

@@ -84,8 +84,12 @@ public void testSerialization() throws IOException {
     }
 
     public void testGetUsedMemoryWithZeroTotal() {
-        OsStats.Mem mem = new OsStats.Mem(0, 1);
+        OsStats.Mem mem = new OsStats.Mem(0, randomLong());
         assertThat(mem.getUsed().getBytes(), equalTo(0L));
     }
 
+    public void testGetUsedSwapWithZeroTotal() {
+        OsStats.Swap swap = new OsStats.Swap(0, randomLong());
+        assertThat(swap.getUsed().getBytes(), equalTo(0L));
+    }
 }

x-pack/plugin/monitoring/src/test/java/org/elasticsearch/xpack/monitoring/collector/node/NodeStatsMonitoringDocTests.java

@@ -330,7 +330,7 @@ private static NodeStats mockNodeStats() {
                 "_memory_ctrl_group", "2000000000", "1000000000");
 
         final OsStats.Mem osMem = new OsStats.Mem(0, 0);
-        final OsStats.Swap osSwap = new OsStats.Swap(no, no);
+        final OsStats.Swap osSwap = new OsStats.Swap(0, 0);
         final OsStats os = new OsStats(no, osCpu, osMem, osSwap, osCgroup);
 
         // Process