ext3: make default data ordering mode configurable

This makes the defautl ext3 data ordering mode (when no explicit
ordering is set) configurable, so as to allow people to default to
'data=writeback' and get the resulting latency improvements.

This is a non-issue if a filesystem has been explicitly set to some
ordering (with 'tune2fs').

Signed-off-by: Linus Torvalds <[e-mail]>

fs/ext3/Kconfig

···
 	  To compile this file system support as a module, choose M here: the
 	  module will be called ext3.
 
+config EXT3_DEFAULTS_TO_ORDERED
+	bool "Default to 'data=ordered' in ext3 (legacy option)"
+	depends on EXT3_FS
+	help
+	  If a filesystem does not explicitly specify a data ordering
+	  mode, and the journal capability allowed it, ext3 used to
+	  historically default to 'data=ordered'.
+
+	  That was a rather unfortunate choice, because it leads to all
+	  kinds of latency problems, and the 'data=writeback' mode is more
+	  appropriate these days.
+
+	  You should probably always answer 'n' here, and if you really
+	  want to use 'data=ordered' mode, set it in the filesystem itself
+	  with 'tune2fs -o journal_data_ordered'.
+
+	  But if you really want to enable the legacy default, you can do
+	  so by answering 'y' to this question.
+
 config EXT3_FS_XATTR
 	bool "Ext3 extended attributes"
 	depends on EXT3_FS

fs/ext3/super.c

···
 #include "acl.h"
 #include "namei.h"
 
+#ifdef CONFIG_EXT3_DEFAULTS_TO_ORDERED
+  #define EXT3_MOUNT_DEFAULT_DATA_MODE EXT3_MOUNT_ORDERED_DATA
+#else
+  #define EXT3_MOUNT_DEFAULT_DATA_MODE EXT3_MOUNT_WRITEBACK_DATA
+#endif
+
 static int ext3_load_journal(struct super_block *, struct ext3_super_block *,
 			     unsigned long journal_devnum);
 static int ext3_create_journal(struct super_block *, struct ext3_super_block *,
···
                    cope, else JOURNAL_DATA */
 		if (journal_check_available_features
 		    (sbi->s_journal, 0, 0, JFS_FEATURE_INCOMPAT_REVOKE))
-			set_opt(sbi->s_mount_opt, ORDERED_DATA);
+			set_opt(sbi->s_mount_opt, DEFAULT_DATA_MODE);
 		else
 			set_opt(sbi->s_mount_opt, JOURNAL_DATA);
 		break;