Re: [RFC PATCH v1.2 15/19] mm/damon/core: implement damon_has_probe_weight()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Enabling probe weights for DAMON operations that do not implement `apply_probes` (e.g., VADDR and FVADDR) silently disables memory monitoring and region aggregation.
--

--- Patch [15]: [RFC PATCH v1.2 15/19] mm/damon/core: implement damon_has_probe_weight() ---
commit 13bb6c9724d49615512e0e4f353c5d02210d2c11
Author: SJ Park <sj@kernel.org>

mm/damon/core: implement damon_has_probe_weight()

This commit completes the implementation of damon_has_probe_weights()
to return true if any probe has a non-zero weight. This allows DAMON
to detect probe weights and use them for monitoring.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 12c66edecd783..6661d4bfab5b9 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -211,6 +211,12 @@ static struct damon_probe *damon_nth_probe(int n, struct damon_ctx *ctx)
>  
>  static bool damon_has_probe_weights(struct damon_ctx *c)
>  {
> +	struct damon_probe *p;
> +
> +	damon_for_each_probe(p, c) {
> +		if (p->weight)
> +			return true;
> +	}

[Severity: High]
Does this silently break monitoring for operation sets that do not implement
apply_probes?

When this function returns true, access_check_disabled evaluates to true in
kdamond_fn():

mm/damon/core.c:kdamond_fn() {
    ...
    bool access_check_disabled = damon_has_probe_weights(ctx);
    ...
    if (!access_check_disabled && ctx->ops.prepare_access_checks)
        ctx->ops.prepare_access_checks(ctx);
    ...
    if (!access_check_disabled && ctx->ops.check_accesses)
        max_merge_score = ctx->ops.check_accesses(ctx);

    if (ctx->ops.apply_probes) {
        ...
    }
    ...
}

If a user configures probe weights for operation sets that lack apply_probes,
all access checks are skipped. Since apply_probes is NULL, no memory access
information is gathered at all. This causes regions to endlessly report 0
accesses and incorrectly merge up to the maximum limit.

Should __damon_commit_ctx() or damon_valid_probe_params() validate that
ctx->ops.apply_probes is non-NULL when probe weights are used, returning
-EINVAL to prevent this failure?

>  	return false;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260709140600.90950-1-sj@kernel.org?part=15