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

Steven Rostedt
2024-01-30T20:55:39Z

On Tue, 30 Jan 2024 11:03:54 -0800
Linus Torvalds <torvalds@linux-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)