Re: [PATCH] mm/damon/sysfs: do not ignore callback's return value in damon_sysfs_damon_call()

[SeongJae Park <sj@kernel.org>]

On Sat, 20 Sep 2025 19:52:36 +0900 Akinobu Mita <akinobu.mita@gmail.com> wrote:

> The callback return value is ignored in damon_sysfs_damon_call(), which
> means that it is not possible to detect invalid user input when writing
> commands such as 'commit' to /sys/kernel/mm/damon/admin/kdamonds/<K>/state.
> Fix it.

Thank you for finding this issue, Akinobu!  Before the introduction of
damon_sysfs_damon_call() and its usages, DAMON sysfs interface was returning an
error in case of such command failures.  So this is fixing a user-visible
unintended behavior change.  Hence this may deserve Cc-ing stable@ in my
opinion.

> 

So, below may better to be added?

Fixes: f64539dcdb87 ("mm/damon/sysfs: use damon_call() for update_schemes_stats")
Cc: <stable@vger.ekrnel.org> # v6.14.x

> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>

Reviewed-by: SeongJae Park <sj@kernel.org>

> ---
>  mm/damon/sysfs.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
> index fe4e73d0ebbb..97e16f00f23e 100644
> --- a/mm/damon/sysfs.c
> +++ b/mm/damon/sysfs.c
> @@ -1627,12 +1627,15 @@ static int damon_sysfs_damon_call(int (*fn)(void *data),
>  		struct damon_sysfs_kdamond *kdamond)
>  {
>  	struct damon_call_control call_control = {};
> +	int err;
>  
>  	if (!kdamond->damon_ctx)
>  		return -EINVAL;
>  	call_control.fn = fn;
>  	call_control.data = kdamond;
> -	return damon_call(kdamond->damon_ctx, &call_control);
> +	err = damon_call(kdamond->damon_ctx, &call_control);
> +

I think the above new line is not really needed, but it is definitely trivial.

> +	return err ? err : call_control.return_code;
>  }
>  
>  struct damon_sysfs_schemes_walk_data {
> -- 
> 2.43.0


Thanks,
SJ