Re: [PATCH 6.6.y v3 1/2] mm/damon/core: do non-safe region walk on kdamond_apply_schemes()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] mm: damon: double processing of split regions by earlier schemes
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commit de615e5101d6a41f308fef487cc464f7d25abb6e
Author: SeongJae Park <sj@kernel.org>

mm/damon/core: do non-safe region walk on kdamond_apply_schemes()

This commit replaces the safe region iterator with damon_for_each_region()
in kdamond_apply_schemes() to ensure newly split regions are not ignored
during iteration. It also simplifies damos_skip_charged_region() by
removing a workaround that updated the region pointer directly.

Link: https://lkml.kernel.org/r/20260227170623.95384-3-sj@kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index fe91b296f4c66..c86cac855b9b0 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -1134,7 +1134,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
>  static void kdamond_apply_schemes(struct damon_ctx *c)
>  {
>  	struct damon_target *t;
> -	struct damon_region *r, *next_r;
> +	struct damon_region *r;
>  	struct damos *s;
>  	unsigned long sample_interval = c->attrs.sample_interval ?
>  		c->attrs.sample_interval : 1;
> @@ -1156,7 +1156,7 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
>  		return;
>  
>  	damon_for_each_target(t, c) {
> -		damon_for_each_region_safe(r, next_r, t)
> +		damon_for_each_region(r, t)
>  			damon_do_apply_schemes(c, t, r);

[Severity: Medium]
Will this change cause earlier schemes to re-process newly split regions?

When damon_do_apply_schemes() evaluates a region, it sequentially iterates
through all schemes. If an earlier scheme processes the region, it applies
its action and updates its quota.

If a later scheme then hits an address filter or quota while evaluating the
same region, it splits the region, shrinking it and inserting a new remainder
region immediately after it in the target list.

Because the outer loop now dynamically fetches the next region via
damon_for_each_region(), this newly inserted remainder region is evaluated
in the very next iteration.

When damon_do_apply_schemes() processes this remainder region, it starts over
from the first scheme. Won't this cause the earlier scheme to re-evaluate
and apply its action a second time, potentially artificially double-charging
its quota and redundantly applying physical memory operations?

>  	}
>

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261009134949.11507-1-sj@kernel.org?part=1