Re: [RFC PATCH 6.12.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/core: inaccurate tried stats on split failure in
  damos_apply_scheme()
--

commit fe019f891ca813eff979dfc83e95ba297bc45135
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 code on
allocation failure and skips applying the DAMOS scheme in
damos_apply_scheme() when splitting a region to fit 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 81d44e41de210..6ef77741b783b 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -1470,7 +1470,8 @@ 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: Low]
Should this return instead of jumping to update_stat when
damon_split_region_at() fails?

At this point sz holds the non-zero aligned remaining quota, and
update_stat calls damos_update_stat(s, sz, sz_applied), which increments
s->stat.nr_tried and adds sz to s->stat.sz_tried even though scheme
application and filter evaluation were skipped.  Because quota->charged_sz
is not updated on this error path, persistent allocation failures across
subsequent regions larger than the remaining quota will repeatedly add the
remaining quota sz to s->stat.sz_tried and increment s->stat.nr_tried.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261107214525.304638-1-sj@kernel.org?part=1