Re: [PATCH v2 3/4] mm/damon: add synchronous commit for commit_inputs

[sashiko-bot@kernel.org]

Sashiko AI review found 1 potential issue(s):
- [High] Writing to the `commit_inputs` sysfs parameter before the DAMON worker thread (`kdamond`) is started causes the writing task to hang indefinitely in an unkillable D-state.
--

commit 43a70e5ab295c21c0b94eddd32d886c9d0eba3fa
Author: Liew Rui Yan <aethernet65535@gmail.com>

mm/damon: add synchronous commit for commit_inputs

This commit changes the commit_inputs_store() callback to use damon_call()
to synchronously commit parameters in the kdamond thread's safe context.
This ensures validation errors are returned immediately to userspace.

Link: https://lore.kernel.org/20260318153731.97470-1-aethernet65535@gmail.com

> diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
> index 554559d72976..641af42cc2d1 100644
> --- a/mm/damon/lru_sort.c
> +++ b/mm/damon/lru_sort.c

[ ... ]

> @@ -349,18 +348,51 @@ static int damon_lru_sort_apply_parameters(void)
>  	return err;
>  }
>  
> -static int damon_lru_sort_handle_commit_inputs(void)
> +static int damon_lru_sort_commit_inputs_fn(void *arg)
>  {
> +	return damon_lru_sort_apply_parameters();
> +}
> +
> +static int damon_lru_sort_commit_inputs_store(const char *val,
> +					      const struct kernel_param *kp)
> +{
> +	bool commit_inputs_request;
>  	int err;
> +	struct damon_call_control control = {
> +		.fn = damon_lru_sort_commit_inputs_fn,
> +	};
>  
> -	if (!commit_inputs)
> +	if (!val) {
> +		commit_inputs_request = true;
> +	} else {
> +		err = kstrtobool(val, &commit_inputs_request);
> +		if (err)
> +			return err;
> +	}
>  
> +	if (!commit_inputs_request)
>  		return 0;
>  
> -	err = damon_lru_sort_apply_parameters();
> -	commit_inputs = false;
> -	return err;
> +	/*
> +	 * Skip damon_call() if ctx is not initialized to avoid
> +	 * NULL pointer dereference.
> +	 */
> +	if (!ctx)
> +		return -EINVAL;
> +
> +	err = damon_call(ctx, &control);

If the module has not yet been enabled (e.g., enabled=N) and the kdamond
thread is not running, does this call to damon_call() block indefinitely?

Since ctx is allocated during initialization, the ctx pointer is not NULL.
damon_call() then queues the control request and calls wait_for_completion().

Without the background thread running to process the queue and signal
completion, it seems the sysfs writer task is placed into an unkillable
D-state forever.

The damon_call() documentation indicates it should only be called after
damon_start() has succeeded, otherwise it could fall into an indefinite wait.

> +
> +	return err ? err : control.return_code;
>  }

[ ... ]

> diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
> index 86da14778658..4fc4a54b5e54 100644
> --- a/mm/damon/reclaim.c
> +++ b/mm/damon/reclaim.c

[ ... ]

> @@ -255,18 +254,51 @@ static int damon_reclaim_apply_parameters(void)
>  	return err;
>  }
>  
> -static int damon_reclaim_handle_commit_inputs(void)
> +static int damon_reclaim_commit_inputs_fn(void *arg)
>  {
> +	return damon_reclaim_apply_parameters();
> +}
> +
> +static int damon_reclaim_commit_inputs_store(const char *val,
> +					     const struct kernel_param *kp)
> +{
> +	bool commit_inputs_request;
>  	int err;
> +	struct damon_call_control control = {
> +		.fn = damon_reclaim_commit_inputs_fn,
> +	};
>  
> -	if (!commit_inputs)
> +	if (!val) {
> +		commit_inputs_request = true;
> +	} else {
> +		err = kstrtobool(val, &commit_inputs_request);
> +		if (err)
> +			return err;
> +	}
> +
> +	if (!commit_inputs_request)
>  		return 0;
>  
> -	err = damon_reclaim_apply_parameters();
> -	commit_inputs = false;
> -	return err;
> +	/*
> +	 * Skip damon_call() if ctx is not initialized to avoid
> +	 * NULL pointer dereference.
> +	 */
> +	if (!ctx)
> +		return -EINVAL;
> +
> +	err = damon_call(ctx, &control);

Could this exhibit the same hang as in damon_lru_sort.c? If the worker
thread has not been started, does writing to commit_inputs block forever?

> +
> +	return err ? err : control.return_code;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260426231619.107231-1-sj@kernel.org?part=3