The tracefs file system was designed from the debugfs file system. The rationale for separating tracefs from debugfs was to allow systems to enable tracing but still keep debugfs disabled. The debugfs API centers around dentry, e.g: struct dentry *debugfs_create_file(const char *name, umode_t mode, struct dentry *parent, void *data, const struct file_operations *fops); struct dentry *debugfs_create_dir(const char *name, struct dentry *parent); Where if you need to create a file in debugfs, you call the above debugfs_create_file() code and it returns a dentry handle, that can be used to delete that file later. If parent is NULL, it adds the file at the root of the debugfs file system (/sys/kernel/debug), otherwise you could create a directory within that file system with the debugfs_create_dir(). Behind the scenes, that dentry also has a created inode structure representing it. This all happens regardless if debugfs is mounted or not! As every trace event in the system is represented by a directory and several files in tracefs's events directory, it created quite a lot of dentries and inodes. # find /sys/kernel/tracing/ | wc -l 18352 And if you create an instance it will duplicate all the events in the instance directory: # mkdir /sys/kernel/tracing/instances/foo # find /sys/kernel/tracing/ | wc -l 36617 And that goes for every new instance you make! # mkdir /sys/kernel/tracing/instances/bar # find /sys/kernel/tracing/ | wc -l 54882 As having inodes and dentries created for all these files and directories even when they are not used, wastes a lot of memory. Two years ago at LSF/MM I presented changing how the events directory works via a new "eventfs" file system. It would still be part of tracefs, but it would dynamically create the inodes and dentries on the fly. As I was new to how VFS works, and really didn't understand it as well as I would have liked, I just got something working and finally submitted it. But because of my inexperience, Linus had some strong issues against the code. Part of this was because I was touching dentries when he said I shouldn't be. But that is because the code was designed from debugfs, which dentry is the central part of that code. When Linus said to me: "And dammit, it shouldn't be necessary. When the tree is mounted, there should be no existing dentries." (I'd share the link, but it was on the security list so there's no public link for this conversation) Linus's comment made me realize how debugfs was doing it wrong! He was right, when a file system is mounted, it should not have any dentries nor inodes. That's because dentry and inodes are basically "cache" of the underlining file system. They should only be created when they are referenced. The debugfs and tracefs (and possibly other pseudo file systems) should not be using dentry as a descriptor for the object. It should just create a generic object that can save the fops, mode, parent, and data, and have the dentries and inodes created when referenced just like any other file system would. Now that I have finished the eventfs file system, I would like to present a proposal to make a more generic interface that the rest of tracefs and even debugfs could use that wouldn't rely on dentry as the main handle. -- Steve
