Re: [RFC PATCH 6.1.y 1/2] mm/damon/core: skip applying scheme if region split for quota fails

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] mm/damon: inflated tried stats in damon_do_apply_schemes() on
  split failure
--

commit 2d405ecddea503227b3b4138fe63cbeb95dc50b9
Author: SJ Park <sj@kernel.org>

mm/damon/core: skip applying scheme if region split for quota fails

This commit changes damon_split_region_at() to return an error on memory
allocation failure and skips applying the DAMOS scheme in
damon_do_apply_schemes() when splitting a region to fit within the size
quota fails.

Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index f0da5cb2b88a0..9206737d05ae7 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -810,7 +810,8 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
>  						DAMON_MIN_REGION);
>  				if (!sz)
>  					goto update_stat;
> -				damon_split_region_at(t, r, sz);
> +				if (damon_split_region_at(t, r, sz))
> +					goto update_stat;

[Severity: Low]
Does jumping to update_stat here inflate s->stat.nr_tried and
s->stat.sz_tried when damon_split_region_at() fails?

Unlike the !sz path right above, sz holds the non-zero remaining quota
when damon_split_region_at() is called, and update_stat unconditionally
accounts it:

mm/damon/core.c:damon_do_apply_schemes() {
	...
update_stat:
		s->stat.nr_tried++;
		s->stat.sz_tried += sz;
	...
}

Because c->ops.apply_scheme() is skipped and quota->charged_sz is not
advanced, any subsequent matching regions in the same pass that also fail
to split will repeatedly add sz to s->stat.sz_tried.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261107224800.340250-1-sj@kernel.org?part=1