fsnotify: Retrieve super block from the data field

Some file system events (i.e. FS_ERROR) might not be associated with an
inode or directory.  For these, we can retrieve the super block from the
data field.  But, since the super_block is available in the data field
on every event type, simplify the code to always retrieve it from there,
through a new helper.

Link: https://lore.kernel.org/r/20211025192746.66445-11-krisman@collabora.com
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fsnotify.c

@@ -455,16 +455,16 @@ static void fsnotify_iter_next(struct fsnotify_iter_info *iter_info)
  *		@file_name is relative to
  * @file_name:	optional file name associated with event
  * @inode:	optional inode associated with event -
- *		either @dir or @inode must be non-NULL.
- *		if both are non-NULL event may be reported to both.
+ *		If @dir and @inode are both non-NULL, event may be
+ *		reported to both.
  * @cookie:	inotify rename cookie
  */
 int fsnotify(__u32 mask, const void *data, int data_type, struct inode *dir,
 	     const struct qstr *file_name, struct inode *inode, u32 cookie)
 {
 	const struct path *path = fsnotify_data_path(data, data_type);
+	struct super_block *sb = fsnotify_data_sb(data, data_type);
 	struct fsnotify_iter_info iter_info = {};
-	struct super_block *sb;
 	struct mount *mnt = NULL;
 	struct inode *parent = NULL;
 	int ret = 0;
@@ -483,7 +483,6 @@ int fsnotify(__u32 mask, const void *data, int data_type, struct inode *dir,
 		 */
 		parent = dir;
 	}
-	sb = inode->i_sb;
 
 	/*
 	 * Optimization: srcu_read_lock() has a memory barrier which can

include/linux/fsnotify_backend.h

@@ -289,6 +289,21 @@ static inline const struct path *fsnotify_data_path(const void *data,
 	}
 }
 
+static inline struct super_block *fsnotify_data_sb(const void *data,
+						   int data_type)
+{
+	switch (data_type) {
+	case FSNOTIFY_EVENT_INODE:
+		return ((struct inode *)data)->i_sb;
+	case FSNOTIFY_EVENT_DENTRY:
+		return ((struct dentry *)data)->d_sb;
+	case FSNOTIFY_EVENT_PATH:
+		return ((const struct path *)data)->dentry->d_sb;
+	default:
+		return NULL;
+	}
+}
+
 enum fsnotify_obj_type {
 	FSNOTIFY_OBJ_TYPE_INODE,
 	FSNOTIFY_OBJ_TYPE_PARENT,