CASSANDRA-21134: Add kernel-bug 1057843 startup-check test

Cover checkKernelBug1057843 for the new background-write Direct I/O data
paths, and tidy the cassandra_latest.yaml comment wording.

Author: samueldlightfoot

conf/cassandra_latest.yaml

@@ -704,8 +704,8 @@ commitlog_disk_access_mode: auto
 # (compaction, streaming, cleanup, repair, etc.). The allowed values are:
 # - standard: use buffered I/O (default)
 # - direct: use direct I/O, bypassing the OS page cache
-# Note: Only applies to compressed tables. Uncompressed tables always use buffered I/O.
-# Note: Memtable flushes always use buffered I/O regardless of this setting, as flushed
+# Only applies to compressed tables. Uncompressed tables always use buffered I/O.
+# Memtable flushes always use buffered I/O regardless of this setting, as flushed
 # data benefits from page cache for subsequent reads.
 background_write_disk_access_mode: direct
 

test/unit/org/apache/cassandra/service/StartupChecksTest.java

@@ -303,12 +303,23 @@ public void testKernelBug1057843Check() throws Exception
     {
         Assume.assumeTrue(DatabaseDescriptor.getCommitLogCompression() == null); // we would not be able to enable direct io otherwise
         Assume.assumeTrue("Skipping this test on non-Linux OS", FBUtilities.isLinux);
-        testKernelBug1057843Check("ext4", DiskAccessMode.direct, new Semver("6.1.63.1-generic"), false);
-        testKernelBug1057843Check("ext4", DiskAccessMode.direct, new Semver("6.1.64.1-generic"), true);
-        testKernelBug1057843Check("ext4", DiskAccessMode.direct, new Semver("6.1.65.1-generic"), true);
-        testKernelBug1057843Check("ext4", DiskAccessMode.direct, new Semver("6.1.66.1-generic"), false);
-        testKernelBug1057843Check("tmpfs", DiskAccessMode.direct, new Semver("6.1.64.1-generic"), false);
-        testKernelBug1057843Check("ext4", DiskAccessMode.mmap, new Semver("6.1.64.1-generic"), false);
+
+        // Commit log direct writes
+        testKernelBug1057843Check("ext4", DiskAccessMode.direct, DiskAccessMode.standard, new Semver("6.1.63.1-generic"), false);
+        testKernelBug1057843Check("ext4", DiskAccessMode.direct, DiskAccessMode.standard, new Semver("6.1.64.1-generic"), true);
+        testKernelBug1057843Check("ext4", DiskAccessMode.direct, DiskAccessMode.standard, new Semver("6.1.65.1-generic"), true);
+        testKernelBug1057843Check("ext4", DiskAccessMode.direct, DiskAccessMode.standard, new Semver("6.1.66.1-generic"), false);
+        testKernelBug1057843Check("tmpfs", DiskAccessMode.direct, DiskAccessMode.standard, new Semver("6.1.64.1-generic"), false);
+        testKernelBug1057843Check("ext4", DiskAccessMode.mmap, DiskAccessMode.standard, new Semver("6.1.64.1-generic"), false);
+
+        // Background (data dir) direct writes hit the same check; kernel/fs-type logic is swept above, so
+        // here we only prove the data-dir branch reaches it (ext4 trips, tmpfs filtered out). Commit log is
+        // mmap, not standard: standard is rejected for the commit log unless compression/encryption is on.
+        testKernelBug1057843Check("ext4", DiskAccessMode.mmap, DiskAccessMode.direct, new Semver("6.1.64.1-generic"), true);
+        testKernelBug1057843Check("tmpfs", DiskAccessMode.mmap, DiskAccessMode.direct, new Semver("6.1.64.1-generic"), false);
+
+        // Both commit log and background data dir direct writes at once
+        testKernelBug1057843Check("ext4", DiskAccessMode.direct, DiskAccessMode.direct, new Semver("6.1.64.1-generic"), true);
     }
 
     @SuppressWarnings("unchecked")
@@ -549,19 +560,25 @@ public FileSystem getFileSystem()
         }
     }
 
-    private void testKernelBug1057843Check(String fsType, DiskAccessMode diskAccessMode, Semver kernelVersion, boolean expectToFail) throws Exception
+    private void testKernelBug1057843Check(String fsType,
+                                           DiskAccessMode commitLogMode,
+                                           DiskAccessMode backgroundMode,
+                                           Semver kernelVersion,
+                                           boolean expectToFail) throws Exception
     {
         String commitLogLocation = Files.createTempDirectory("testKernelBugCheck").toString();
 
         String savedCommitLogLocation = DatabaseDescriptor.getCommitLogLocation();
         DiskAccessMode savedCommitLogWriteDiskAccessMode = DatabaseDescriptor.getCommitLogWriteDiskAccessMode();
+        DiskAccessMode savedBackgroundWriteDiskAccessMode = DatabaseDescriptor.getBackgroundWriteDiskAccessMode();
         SystemInfo savedSystemInfo = FBUtilities.getSystemInfo();
         try
         {
+            DatabaseDescriptor.setBackgroundWriteDiskAccessMode(backgroundMode);
             DatabaseDescriptor.setCommitLogLocation(commitLogLocation);
-            DatabaseDescriptor.setCommitLogWriteDiskAccessMode(diskAccessMode);
+            DatabaseDescriptor.setCommitLogWriteDiskAccessMode(commitLogMode);
             DatabaseDescriptor.initializeCommitLogDiskAccessMode();
-            assertThat(DatabaseDescriptor.getCommitLogWriteDiskAccessMode()).isEqualTo(diskAccessMode);
+            assertThat(DatabaseDescriptor.getCommitLogWriteDiskAccessMode()).isEqualTo(commitLogMode);
             FBUtilities.setSystemInfoSupplier(() -> new SystemInfo()
             {
                 @Override
@@ -570,7 +587,15 @@ public Semver getKernelVersion()
                     return kernelVersion;
                 }
             });
-            withPathOverriddingFileSystem(Map.of(commitLogLocation, fsType), () -> {
+
+            // Override the filesystem type for every path getDirectIOWritePaths() may return (commit log and
+            // all data dirs) so the check sees only our synthetic type, never the real test filesystem.
+            Map<String, String> pathOverrides = new HashMap<>();
+            pathOverrides.put(commitLogLocation, fsType);
+            for (String dataDir : DatabaseDescriptor.getAllDataFileLocations())
+                pathOverrides.put(dataDir, fsType);
+
+            withPathOverriddingFileSystem(pathOverrides, () -> {
                 if (expectToFail)
                     assertThatExceptionOfType(StartupException.class).isThrownBy(() -> StartupChecks.checkKernelBug1057843.execute(options));
                 else
@@ -583,6 +608,7 @@ public Semver getKernelVersion()
             DatabaseDescriptor.setCommitLogLocation(savedCommitLogLocation);
             DatabaseDescriptor.setCommitLogWriteDiskAccessMode(savedCommitLogWriteDiskAccessMode);
             DatabaseDescriptor.initializeCommitLogDiskAccessMode();
+            DatabaseDescriptor.setBackgroundWriteDiskAccessMode(savedBackgroundWriteDiskAccessMode);
             FBUtilities.setSystemInfoSupplier(() -> savedSystemInfo);
         }
     }