Re: [RFC PATCH] mm/damon: reset thread status parameters upon kdamond termination

[Liew Rui Yan <aethernet65535@gmail.com>]

Hi SeongJae,

On Tue, 31 Mar 2026 14:58:36 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:

> [...]
> Option 1: Introduce a generic termination callback
> ==================================================
> Add 'void *after_terminate_fn(void*)' and 'void *after_terminate_data'
> to 'struct damon_ctx' or 'struct damon_operations'. While this extends
> the Core API, it provides a clean notification mechanism. When kdamond
> exits for any reason, the module can perform its own cleanup (e.g.,
> resetting 'enabled' and 'kdamond_pid') within its own callback. This
> keeps the core logic decoupled from module parameters.
> 
> Option 2: On-demand state correction in the module
> ==================================================
> In damon_{lru_sort, reclaim}_enabled_store(), if damon_stop() fails, we
> check is_kdamond_running(). If the kdamond is found to be terminated, we
> forcibly reset 'enabled' and 'kdamond_pid'.
> 
> My perspective
> --------------
> I personally prefer OPTION-1 because it ensures the sysfs state in
> synchronized actively.
> 
> OPTION-2 is simpler and avoids API changes, but it's a "passive" fix
> that only triggers when user atttempts a write operation. User might
> still see inconsistent value until they try to interact with the module.
> [...]

To avoid over-complicating the Core API (Option 1 or current approach),
I've implemented a lightweight, on-demand synchronization mechanism in
the module layer.

By overriding the '.get' operator of the 'enabled' parameter, we can
detect if kdamond has terminated and reset the module-level state right
before the user reads them.

Since sysfs parameter access is protected by 'kernel_param_lock', this
approach is race-free and keeps the DAMON core decoupling intact.

Core Implementation:

	if (enabled && !damon_is_running(ctx)) {
		enabled = false;
		kdamond_pid = -1;
	}

	return param_get_bool(buffer, kp);

Test Log:

    # cd /sys/module/damon_lru_sort/parameters/
    # echo Y > enabled 
    # echo 3 > addr_unit 
    # echo Y > commit_inputs 
    # cat enabled 
    N
    # cat kdamond_pid 
    -1

I think this approach is better than my previous OPTION-1 and OPTION-2.
Does this looks good to you?

Best regards,
Rui Yan