ref_tracker: add a way to create a symlink to the ref_tracker_dir debugfs file

Add the ability for a subsystem to add a user-friendly symlink that
points to a ref_tracker_dir's debugfs file. Add a separate
debugfs_symlinks xarray and use that to track symlinks. The reaper
workqueue job will remove symlinks before their corresponding dentries.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20250618-reftrack-dbgfs-v15-7-24fc37ead144@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 61 insertions, 0 deletions

diff --git a/include/linux/ref_tracker.h b/include/linux/ref_tracker.h
index 28bbf436a8f4..e1323de93bf6 100644
--- a/include/linux/ref_tracker.h
+++ b/include/linux/ref_tracker.h
@@ -29,6 +29,7 @@ struct ref_tracker_dir {
 #ifdef CONFIG_DEBUG_FS
 
 void ref_tracker_dir_debugfs(struct ref_tracker_dir *dir);
+void ref_tracker_dir_symlink(struct ref_tracker_dir *dir, const char *fmt, ...);
 
 #else /* CONFIG_DEBUG_FS */
 
@@ -36,6 +37,11 @@ static inline void ref_tracker_dir_debugfs(struct ref_tracker_dir *dir)
 {
 }
 
+static inline __ostream_printf
+void ref_tracker_dir_symlink(struct ref_tracker_dir *dir, const char *fmt, ...)
+{
+}
+
 #endif /* CONFIG_DEBUG_FS */
 
 static inline void ref_tracker_dir_init(struct ref_tracker_dir *dir,
@@ -85,6 +91,11 @@ static inline void ref_tracker_dir_debugfs(struct ref_tracker_dir *dir)
 {
 }
 
+static inline __ostream_printf
+void ref_tracker_dir_symlink(struct ref_tracker_dir *dir, const char *fmt, ...)
+{
+}
+
 static inline void ref_tracker_dir_exit(struct ref_tracker_dir *dir)
 {
 }
diff --git a/lib/ref_tracker.c b/lib/ref_tracker.c
index c938ef56954b..6608520d6118 100644
--- a/lib/ref_tracker.c
+++ b/lib/ref_tracker.c
@@ -44,6 +44,7 @@ struct ref_tracker_dir_stats {
  * dentries asynchronously.
  */
 static struct xarray		debugfs_dentries;
+static struct xarray		debugfs_symlinks;
 static struct work_struct	debugfs_reap_worker;
 
 #define REF_TRACKER_DIR_DEAD	XA_MARK_0
@@ -55,6 +56,10 @@ static inline void ref_tracker_debugfs_mark(struct ref_tracker_dir *dir)
 	__xa_set_mark(&debugfs_dentries, (unsigned long)dir, REF_TRACKER_DIR_DEAD);
 	xa_unlock_irqrestore(&debugfs_dentries, flags);
 
+	xa_lock_irqsave(&debugfs_symlinks, flags);
+	__xa_set_mark(&debugfs_symlinks, (unsigned long)dir, REF_TRACKER_DIR_DEAD);
+	xa_unlock_irqrestore(&debugfs_symlinks, flags);
+
 	schedule_work(&debugfs_reap_worker);
 }
 #else
@@ -451,6 +456,45 @@ void ref_tracker_dir_debugfs(struct ref_tracker_dir *dir)
 }
 EXPORT_SYMBOL(ref_tracker_dir_debugfs);
 
+void __ostream_printf ref_tracker_dir_symlink(struct ref_tracker_dir *dir, const char *fmt, ...)
+{
+	char name[NAME_MAX + 1];
+	struct dentry *symlink, *dentry;
+	va_list args;
+	int ret;
+
+	symlink = xa_load(&debugfs_symlinks, (unsigned long)dir);
+	dentry = xa_load(&debugfs_dentries, (unsigned long)dir);
+
+	/* Already created?*/
+	if (symlink && !xa_is_err(symlink))
+		return;
+
+	if (!dentry || xa_is_err(dentry))
+		return;
+
+	va_start(args, fmt);
+	ret = vsnprintf(name, sizeof(name), fmt, args);
+	va_end(args);
+	name[sizeof(name) - 1] = '\0';
+
+	if (ret < sizeof(name)) {
+		symlink = debugfs_create_symlink(name, ref_tracker_debug_dir,
+						 dentry->d_name.name);
+		if (!IS_ERR(symlink)) {
+			void *old;
+
+			old = xa_store_irq(&debugfs_symlinks, (unsigned long)dir,
+					   symlink, GFP_KERNEL);
+			if (xa_is_err(old))
+				debugfs_remove(symlink);
+			else
+				WARN_ON_ONCE(old);
+		}
+	}
+}
+EXPORT_SYMBOL(ref_tracker_dir_symlink);
+
 static void debugfs_reap_work(struct work_struct *work)
 {
 	struct dentry *dentry;
@@ -459,6 +503,11 @@ static void debugfs_reap_work(struct work_struct *work)
 
 	do {
 		reaped = false;
+		xa_for_each_marked(&debugfs_symlinks, index, dentry, REF_TRACKER_DIR_DEAD) {
+			xa_erase_irq(&debugfs_symlinks, index);
+			debugfs_remove(dentry);
+			reaped = true;
+		}
 		xa_for_each_marked(&debugfs_dentries, index, dentry, REF_TRACKER_DIR_DEAD) {
 			xa_erase_irq(&debugfs_dentries, index);
 			debugfs_remove(dentry);
@@ -471,6 +520,7 @@ static int __init ref_tracker_debugfs_init(void)
 {
 	INIT_WORK(&debugfs_reap_worker, debugfs_reap_work);
 	xa_init_flags(&debugfs_dentries, XA_FLAGS_LOCK_IRQ);
+	xa_init_flags(&debugfs_symlinks, XA_FLAGS_LOCK_IRQ);
 	ref_tracker_debug_dir = debugfs_create_dir("ref_tracker", NULL);
 	return 0;
 }