Re: (sashiko review) [PATCH 2/2] Docs/admin-guide/mm/damon/lru_sort: warn commit_inputs vs param updates race

[SeongJae Park <sj@kernel.org>]

Forwarding Sashiko review for doing discussions via mails.

# review url: https://sashiko.dev/#/patchset/20260329153052.46657-3-sj@kernel.org
# start of sashiko.dev inline review
commit 7f5e14b7be3b4aea36d62a6c529ef25dde22e973
Author: SeongJae Park <sj@kernel.org>

Docs/admin-guide/mm/damon/lru_sort: warn commit_inputs vs param updates race

This commit updates the DAMON LRU_SORT documentation to warn users against
modifying module parameters while the kdamond thread is processing a
commit_inputs request. It states that violating this rule can cause races
and undefined behavior in the kernel.

Link: https://lore.kernel.org/20260319161620.189392-2-objecting@objecting.org

> diff --git a/Documentation/admin-guide/mm/damon/lru_sort.rst b/Documentation/admin-guide/mm/damon/lru_sort.rst
> index a7dea7c75a9b..14cc6b2db897 100644
> --- a/Documentation/admin-guide/mm/damon/lru_sort.rst
> +++ b/Documentation/admin-guide/mm/damon/lru_sort.rst
> @@ -79,6 +79,10 @@ of parametrs except ``enabled`` again.  Once the re-reading is done, this
>  parameter is set as ``N``.  If invalid parameters are found while the
>  re-reading, DAMON_LRU_SORT will be disabled.
>  
> +Once ``Y`` is written to this parameter, the user must not write to any
> +parameters until reading ``commit_inputs`` again returns ``N``.  If users
> +violate this rule, the kernel may exhibit undefined behavior.

Is it safe to rely on userspace following documentation to prevent kernel
data races?

The kernel is generally expected to be robust against concurrent or unexpected
userspace inputs. Does addressing a data race with a documentation warning
leave the kernel vulnerable to crashes or instability if a user ignores this
rule?

Could this underlying race condition be resolved with proper synchronization,
such as locks or mutexes, rather than documenting the undefined behavior?


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