Re: [RFC PATCH 6.6.y 1/3] 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:
- [Medium] mm/damon: false tried stats on split failure in
  damos_apply_scheme()
--

commit ae5a2312a946a6108ba6414ca8a1f6fb984dd078
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 remaining
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 b5b6d00d7a3b5..9d4a78deaa06a 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -1004,7 +1004,8 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
>  					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;
>  		}
>  		if (damos_filter_out(c, t, r, s))
>  			return;

[Severity: Medium]
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 is non-zero, so damos_update_stat(s, sz, sz_applied)
increments s->stat.nr_tried and adds the remaining quota size sz to
s->stat.sz_tried even though scheme application was skipped before
damos_filter_out() or c->ops.apply_scheme() ran.  Because quota->charged_sz
is not updated either, every subsequent matching region in the same walk
that fails to split will add sz to s->stat.sz_tried again.  Returning
directly instead of jumping to update_stat would avoid accounting the
skipped region as tried.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261107222202.331476-1-sj@kernel.org?part=1