[PATCH v3] btrfs: add simple debugfs interface

[David Sterba <dsterba@suse.cz>]

Help during debugging to export various interesting information and
tunables without the need of extra mount options or ioctls.

Usage:
* declare your variable in sysfs.h, and include where you need it
* define the variable in sysfs.c and make it visible via
  debugfs_create_TYPE

Depends on CONFIG_DEBUG_FS.

Signed-off-by: David Sterba <dsterba@suse.cz>
---

v3:
- fix typo in changelog

v2:
- added missing return to btrfs_init_debugfs
- updated error handling to btrfs_init_sysfs, the cleanup
  is done in btrfs_exit_sysfs
- removed #ifdef in btrfs_exit_sysfs,

 fs/btrfs/sysfs.c |   33 +++++++++++++++++++++++++++------
 1 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 782374d8fd19..b725e4574448 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -24,6 +24,7 @@
 #include <linux/kobject.h>
 #include <linux/bug.h>
 #include <linux/genhd.h>
+#include <linux/debugfs.h>
 
 #include "ctree.h"
 #include "disk-io.h"
@@ -593,6 +594,12 @@ static int add_device_membership(struct btrfs_fs_info *fs_info)
 /* /sys/fs/btrfs/ entry */
 static struct kset *btrfs_kset;
 
+/* /sys/kernel/debug/btrfs */
+static struct dentry *btrfs_debugfs_root_dentry;
+
+/* Debugging tunables and exported data */
+u64 btrfs_debugfs_test;
+
 int btrfs_sysfs_add_one(struct btrfs_fs_info *fs_info)
 {
 	int error;
@@ -636,27 +643,41 @@ failure:
 	return error;
 }
 
+static int btrfs_init_debugfs(void)
+{
+#ifdef CONFIG_DEBUG_FS
+	btrfs_debugfs_root_dentry = debugfs_create_dir("btrfs", NULL);
+	if (!btrfs_debugfs_root_dentry)
+		return -ENOMEM;
+
+	debugfs_create_u64("test", S_IRUGO | S_IWUGO, btrfs_debugfs_root_dentry,
+			&btrfs_debugfs_test);
+#endif
+	return 0;
+}
+
 int btrfs_init_sysfs(void)
 {
 	int ret;
+
 	btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
 	if (!btrfs_kset)
 		return -ENOMEM;
 
-	init_feature_attrs();
+	ret = btrfs_init_debugfs();
+	if (ret)
+		return ret;
 
+	init_feature_attrs();
 	ret = sysfs_create_group(&btrfs_kset->kobj, &btrfs_feature_attr_group);
-	if (ret) {
-		kset_unregister(btrfs_kset);
-		return ret;
-	}
 
-	return 0;
+	return ret;
 }
 
 void btrfs_exit_sysfs(void)
 {
 	sysfs_remove_group(&btrfs_kset->kobj, &btrfs_feature_attr_group);
 	kset_unregister(btrfs_kset);
+	debugfs_remove_recursive(btrfs_debugfs_root_dentry);
 }
 
-- 
1.7.9