Re: [PATCH 2/9] ftrace: Add register_ftrace_direct_hash function

On Wed, Sep 24, 2025 at 05:04:15AM -0400, Steven Rostedt wrote:

On Tue, 23 Sep 2025 23:51:40 +0200
Jiri Olsa [off-list ref] wrote:

quoted

Adding register_ftrace_direct_hash function that registers
all entries (ip -> direct) provided in hash argument.

The difference to current register_ftrace_direct is
 - hash argument that allows to register multiple ip -> direct
   entries at once

I'm a bit confused. How is this different? Doesn't
register_ftrace_direct() register multiple ip -> direct entries at once
too? But instead of using a passed in hash, it uses the hash from
within the ftrace_ops.

right, but that assumes that we can touch the hash in ftrace_ops directly,
but register_ftrace_direct_hash semantics is bit different, because it allows
to register new (ip,addr) entries on already 'running' ftrace_ops, in which
case you can't change the ftrace_ops hash directly

quoted

 - we can call register_ftrace_direct_hash multiple times on the
   same ftrace_ops object, becase after first registration with
   register_ftrace_function_nolock, it uses ftrace_update_ops to
   update the ftrace_ops object

OK, I don't like the name "register" here. "register" should be for the
first instance and then it is registered. If you call it multiple times
on the same ops without "unregister" it should give an error.

Perhaps call this "update_ftrace_direct()" where it can update a direct
ftrace_ops from?

I agree the 'register' naming is confusing in here.. but we still need to
use 3 functions for register/unregister/modify operations, so perhaps:

   update_ftrace_direct_add(ops, hash)
   update_ftrace_direct_del(ops, hash)
   update_ftrace_direct_mod(ops, hash)

?

quoted

This change will allow us to have simple ftrace_ops for all bpf
direct interface users in following changes.

After applying all the patches, I have this:

$ git grep register_ftrace_direct_hash
include/linux/ftrace.h:int register_ftrace_direct_hash(struct ftrace_ops *ops, struct ftrace_hash *hash);
include/linux/ftrace.h:int unregister_ftrace_direct_hash(struct ftrace_ops *ops, struct ftrace_hash *hash);
include/linux/ftrace.h:int register_ftrace_direct_hash(struct ftrace_ops *ops, struct ftrace_hash *hash)
include/linux/ftrace.h:int unregister_ftrace_direct_hash(struct ftrace_ops *ops, struct ftrace_hash *hash)
kernel/trace/ftrace.c:  err = register_ftrace_direct_hash(ops, hash);
kernel/trace/ftrace.c:  err = unregister_ftrace_direct_hash(ops, hash);
kernel/trace/ftrace.c:int register_ftrace_direct_hash(struct ftrace_ops *ops, struct ftrace_hash *hash)
kernel/trace/ftrace.c:EXPORT_SYMBOL_GPL(register_ftrace_direct_hash);
kernel/trace/ftrace.c:int unregister_ftrace_direct_hash(struct ftrace_ops *ops, struct ftrace_hash *hash)
kernel/trace/ftrace.c:EXPORT_SYMBOL_GPL(unregister_ftrace_direct_hash);

Where I do not see it is used outside of ftrace.c. Why is it exported?

I have bpf changes using this that I did not post yet, but even with that
there's probably no reason to export this.. will remove

thanks,
jirka