Re: [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 17459070b1916b2d77f607f0fb7f0f9158e6224f
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() if 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 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;

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

At this point sz is the non-zero remaining quota size, and
damos_update_stat(s, sz, sz_applied) unconditionally 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 split failure, every subsequent
matching region in the walk that exceeds the remaining quota and fails to
split will also add sz to s->stat.sz_tried again.  Returning early here, as
damos_filter_out() does just below, would avoid counting skipped regions as
tried.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261108201456.116824-1-sj@kernel.org?part=1