linux-kernel - Re: [PATCH 5/6] eventfs: get rid of dentry pointers without refcounts

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Date: Tue, 30 Jan 2024 15:55:50 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] eventfs: get rid of dentry pointers without
 refcounts

On Tue, 30 Jan 2024 11:03:54 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> -static void free_ei(struct eventfs_inode *ei)
> +static inline struct eventfs_inode *alloc_ei(const char *name)
>  {
> -	kfree_const(ei->name);
> -	kfree(ei->d_children);
> -	kfree(ei->entry_attrs);
> -	kfree(ei);
> +	int namesize = strlen(name) + 1;
> +	struct eventfs_inode *ei = kzalloc(sizeof(*ei) + namesize, GFP_KERNEL);
> +
> +	if (ei) {
> +		memcpy((char *)ei->name, name, namesize);
> +		kref_init(&ei->kref);
> +	}

I'm going to be putting back the ei->name pointer as the above actually
adds more memory usage. The reason being is that all static trace events
(not kprobes or other dynamic events) are created with the TRACE_EVENT()
macro that points to a constant name. Passing in a const to kstrdup_const()
checks if the name passed in is a constant or not. If it is, it doesn't
allocate the name and just passes back the pointer.

Removing the pointer to "name" removed 8 bytes on 64 bit machines from the
eventfs_inode structure, but added strlen(name)+1 bytes to it, where the
majority of trace event names are greater than 8 bytes, and are constant
strings.

Another reason for converting back to the way it was is I have moving the
ei allocation to a slab on my TODO list.

-- Steve



> +	return ei;
>  }
>  


> diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
> index 8f38740bfb5b..72db3bdc4dfb 100644
> --- a/fs/tracefs/internal.h
> +++ b/fs/tracefs/internal.h
> @@ -34,8 +34,7 @@ struct eventfs_attr {
>   * @entries:	the array of entries representing the files in the directory
>   * @name:	the name of the directory to create
>   * @children:	link list into the child eventfs_inode
> - * @dentry:     the dentry of the directory
> - * @d_children: The array of dentries to represent the files when created
> + * @events_dir: the dentry of the events directory
>   * @entry_attrs: Saved mode and ownership of the @d_children
>   * @attr:	Saved mode and ownership of eventfs_inode itself
>   * @data:	The private data to pass to the callbacks
> @@ -44,12 +43,11 @@ struct eventfs_attr {
>   * @nr_entries: The number of items in @entries
>   */
>  struct eventfs_inode {
> +	struct kref			kref;
>  	struct list_head		list;
>  	const struct eventfs_entry	*entries;
> -	const char			*name;
>  	struct list_head		children;
> -	struct dentry			*dentry; /* Check is_freed to access */
> -	struct dentry			**d_children;
> +	struct dentry			*events_dir;
>  	struct eventfs_attr		*entry_attrs;
>  	struct eventfs_attr		attr;
>  	void				*data;
> @@ -66,6 +64,7 @@ struct eventfs_inode {
>  		struct llist_node	llist;
>  		struct rcu_head		rcu;
>  	};
> +	const char name[];
>  };
>  
>  static inline struct tracefs_inode *get_tracefs(const struct inode *inode)