Re: [RFC PATCH 6.18.y 1/5] 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: damos_apply_scheme() inflates stats on region split
  failure
--

commit dd2b638af6fec217ed7123ab31fe119102f17f78
Author: SJ Park <sj@kernel.org>

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

This commit updates damon_split_region_at() to return an error code on
allocation failure and skips applying the DAMOS scheme in
damos_apply_scheme() 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 8fa788b0d709c..4c536a9b958e4 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
[ ... ]
> @@ -2000,7 +2000,8 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
>  					c->min_sz_region);
>  			if (!sz)
>  				goto update_stat;
> -			damon_split_region_at(t, r, sz);
> +			if (damon_split_region_at(t, r, sz))
> +				goto update_stat;
>  		}
>  		if (damos_filter_out(c, t, r, s))
>  			return;

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

At this point sz holds the non-zero aligned quota remainder, so
damos_update_stat() increments s->stat.nr_tried and adds sz to
s->stat.sz_tried even though damos_filter_out() and c->ops.apply_scheme()
were skipped.  Because quota->charged_sz is not updated on this failure
path, subsequent oversized matching regions in the walk will repeatedly add
sz to s->stat.sz_tried while allocation keeps failing.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261107201001.206120-1-sj@kernel.org?part=1