Re: [PATCH] mm/damon: reject zero sampling interval

[Sang-Heon Jeon]

Hi,

On Fri, Sep 18, 2026 at 12:14 AM Karthikeyan KS
<karthiproffesional@xxxxxxxxx> wrote:
>
> damon_set_attrs() already rejects inverted intervals (sample_interval >
> aggr_interval) and invalid region counts, but it accepts
> sample_interval == 0.
>
> A zero sampling interval is not a valid monitoring period.  Sysfs "on"
> and "commit" both install attrs via damon_set_attrs(), so kdamond then
> calls kdamond_usleep(0) and busy-spins.
>
> Return -EINVAL if sample_interval is zero.

I think SJ already commented on a similar patch. please refer to [1]
If this happens a lot, maybe should we document or add a comment about this?

[1] https://lore.kernel.org/all/20260722094304.3132750-1-dayou5941@xxxxxxx/

> Signed-off-by: Karthikeyan KS <karthiproffesional@xxxxxxxxx>
> ---
>
> Hello,
>
> I recently started reading DAMON and just found this one.  This
> change only tightens damon_set_attrs() validation, in the same place
> as the existing sample > aggr and min_nr_regions checks.  It is not a
> crash, UAF, or race.
>
> sample_us=0 is accepted today, so kdamond_usleep(0) busy-spins (about
> one core).  Sysfs "on" and "commit" both go through damon_set_attrs();
> one check covers both.
>
> Checked on v7.3-rc3 (238650ef6c7c):
>
>   unpatched: echo on with sample_us=0 succeeds; kdamond uses ~97% of
>              one core.  echo commit of 0 onto a running kdamond also
>              succeeds and it starts spinning.
>
>   patched:   both on and commit return -EINVAL.  Valid sample_us=5000
>              still starts.  After a rejected commit the same pid stays
>              on and does not spin.
>
> aggr_us=0 with a positive sample_us is already -EINVAL from the
> ordering check.
>
> Thanks,
>
>  mm/damon/core.c             |  2 ++
>  mm/damon/tests/core-kunit.h | 13 +++++++++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 644daf5a1656..376c3e2cbe00 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -1060,6 +1060,8 @@ int damon_set_attrs(struct damon_ctx *ctx, struct damon_attrs *attrs)
>                 return -EINVAL;
>         if (attrs->min_nr_regions > attrs->max_nr_regions)
>                 return -EINVAL;
> +       if (!attrs->sample_interval)
> +               return -EINVAL;
>         if (attrs->sample_interval > attrs->aggr_interval)
>                 return -EINVAL;
>
> diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
> index 4a536d41cdb2..ec8ef71956c8 100644
> --- a/mm/damon/tests/core-kunit.h
> +++ b/mm/damon/tests/core-kunit.h
> @@ -658,6 +658,19 @@ static void damon_test_set_attrs(struct kunit *test)
>         invalid_attrs.aggr_interval = 4999;
>         KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
>
> +       invalid_attrs = valid_attrs;
> +       invalid_attrs.sample_interval = 0;
> +       KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
> +
> +       invalid_attrs = valid_attrs;
> +       invalid_attrs.aggr_interval = 0;
> +       KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
> +
> +       invalid_attrs = valid_attrs;
> +       invalid_attrs.sample_interval = 0;
> +       invalid_attrs.aggr_interval = 0;
> +       KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
> +
>         damon_destroy_ctx(c);
>  }
>
> --
> 2.34.1
>

Best regards,
Sang-Heon Jeon