Re: (sashiko review) [PATCH v2 1/2] mm/damon/lru_sort: reset thread status parameters upon kdamond termination

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

Sashiko found some issues, so I will send my comment for it soon.

Forwarding sashiko review for doing discussions via mails.

# review url: https://sashiko.dev/#/patchset/20260413185249.5921-2-aethernet65535@gmail.com
# start of sashiko.dev inline review
commit a943fcfe60e73956b72cad5a625120222653bdda
Author: Liew Rui Yan <aethernet65535@gmail.com>

mm/damon/lru_sort: reset thread status parameters upon kdamond termination

This commit adds logic to reset thread status parameters when kdamond
terminates unexpectedly or when context commit fails. This ensures the
module does not get stuck in a state where it cannot be restarted via sysfs.

> diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
> index 554559d729760..96c8d0dfcafd2 100644
> --- a/mm/damon/lru_sort.c
> +++ b/mm/damon/lru_sort.c
> @@ -344,6 +344,10 @@ static int damon_lru_sort_apply_parameters(void)
>  	if (err)
>  		goto out;
>  	err = damon_commit_ctx(ctx, param_ctx);
> +	if (err) {
> +		enabled = false;
> +		kdamond_pid = -1;
> +	}

Does updating the module parameters here require holding kernel_param_lock?

Since damon_lru_sort_apply_parameters() can be executed asynchronously by the
kdamond thread when a user writes to the commit_inputs parameter, changing
these variables locklessly might introduce a race condition.

If enabled is set to false here while kdamond is still preparing to terminate,
could a concurrent sysfs write (echo Y > enabled) read the false state and
proceed to call damon_lru_sort_turn(true) because it incorrectly assumes the
worker has completely stopped?

If so, damon_lru_sort_turn(true) would call damon_commit_ctx(ctx, ...) and
modify the shared ctx structures while the exiting kdamond worker thread is
concurrently executing its cleanup block, such as damon_destroy_targets(ctx).

Can this concurrent modification of the context lists lead to use-after-free
issues or list corruption?

[ ... ]


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