Re: [for-next][PATCH 3/7] tracefs: Add new tracefs file system

[Namhyung Kim <namhyung@kernel.org>]

Hi Steve,

On Wed, Feb 04, 2015 at 09:34:23AM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
> 
> Add a separate file system to handle the tracing directory. Currently it
> is part of debugfs, but that is starting to show its limits.
> 
> One thing is that in order to access the tracing infrastructure, you need
> to mount debugfs. As that includes debugging from all sorts of sub systems
> in the kernel, it is not considered advisable to mount such an all
> encompassing debugging system.
> 
> Having the tracing system in its own file systems gives access to the
> tracing sub system without needing to include all other systems.
> 
> Another problem with tracing using the debugfs system is that the
> instances use mkdir to create sub buffers. debugfs does not support mkdir
> from userspace so to implement it, special hacks were used. By controlling
> the file system that the tracing infrastructure uses, this can be properly
> done without hacks.
> 
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---

[SNIP]
> +/**
> + * tracefs_create_file - create a file in the tracefs filesystem
> + * @name: a pointer to a string containing the name of the file to create.
> + * @mode: the permission that the file should have.
> + * @parent: a pointer to the parent dentry for this file.  This should be a
> + *          directory dentry if set.  If this parameter is NULL, then the
> + *          file will be created in the root of the tracefs filesystem.
> + * @data: a pointer to something that the caller will want to get to later
> + *        on.  The inode.i_private pointer will point to this value on
> + *        the open() call.
> + * @fops: a pointer to a struct file_operations that should be used for
> + *        this file.
> + *
> + * This is the basic "create a file" function for tracefs.  It allows for a
> + * wide range of flexibility in creating a file, or a directory (if you want
> + * to create a directory, the tracefs_create_dir() function is
> + * recommended to be used instead.)
> + *
> + * This function will return a pointer to a dentry if it succeeds.  This
> + * pointer must be passed to the tracefs_remove() function when the file is
> + * to be removed (no automatic cleanup happens if your module is unloaded,
> + * you are responsible here.)  If an error occurs, %NULL will be returned.
> + *
> + * If tracefs is not enabled in the kernel, the value -%ENODEV will be
> + * returned.

I cannot find where it returns -ENODEV.  AFAICS we cannot call tracefs
functions if tracing was not enabled.


> + */
> +struct dentry *tracefs_create_file(const char *name, umode_t mode,
> +				   struct dentry *parent, void *data,
> +				   const struct file_operations *fops)
> +{
> +	struct dentry *dentry;
> +	struct inode *inode;
> +
> +	if (!(mode & S_IFMT))
> +		mode |= S_IFREG;
> +	BUG_ON(!S_ISREG(mode));
> +	dentry = start_creating(name, parent);
> +
> +	if (IS_ERR(dentry))
> +		return NULL;
> +
> +	inode = tracefs_get_inode(dentry->d_sb);
> +	if (unlikely(!inode))
> +		return failed_creating(dentry);
> +
> +	inode->i_mode = mode;
> +	inode->i_fop = fops ? fops : &tracefs_file_operations;
> +	inode->i_private = data;
> +	d_instantiate(dentry, inode);
> +	fsnotify_create(dentry->d_parent->d_inode, dentry);
> +	return end_creating(dentry);
> +}
> +
> +/**
> + * tracefs_create_dir - create a directory in the tracefs filesystem
> + * @name: a pointer to a string containing the name of the directory to
> + *        create.
> + * @parent: a pointer to the parent dentry for this file.  This should be a
> + *          directory dentry if set.  If this parameter is NULL, then the
> + *          directory will be created in the root of the tracefs filesystem.
> + *
> + * This function creates a directory in tracefs with the given name.
> + *
> + * This function will return a pointer to a dentry if it succeeds.  This
> + * pointer must be passed to the tracefs_remove() function when the file is
> + * to be removed. If an error occurs, %NULL will be returned.
> + *
> + * If tracing is not enabled in the kernel, the value -%ENODEV will be
> + * returned.

Ditto.

Thanks,
Namhyung


> + */
> +struct dentry *tracefs_create_dir(const char *name, struct dentry *parent)
> +{
> +	struct dentry *dentry = start_creating(name, parent);
> +	struct inode *inode;
> +
> +	if (IS_ERR(dentry))
> +		return NULL;
> +
> +	inode = tracefs_get_inode(dentry->d_sb);
> +	if (unlikely(!inode))
> +		return failed_creating(dentry);
> +
> +	inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
> +	inode->i_op = &simple_dir_inode_operations;
> +	inode->i_fop = &simple_dir_operations;
> +
> +	/* directory inodes start off with i_nlink == 2 (for "." entry) */
> +	inc_nlink(inode);
> +	d_instantiate(dentry, inode);
> +	inc_nlink(dentry->d_parent->d_inode);
> +	fsnotify_mkdir(dentry->d_parent->d_inode, dentry);
> +	return end_creating(dentry);
> +}