[PATCH v2 3/3] Docs/admin-guide/mm/damon/usage: document the temporal tuner's quota limit

[PATCH v2 3/3] Docs/admin-guide/mm/damon/usage: document the temporal tuner's quota limit

[Donggeun Yoo]

The temporal goal tuner converts the size quota into basis points in an
unsigned long, so a quotas/bytes above ULONG_MAX / 10000 does not fit,
and is handled as if no size quota were set.  On 32-bit that is 429496
bytes, well below the "echo $((1024*1024*1024)) > quotas/bytes" this
document instructs.  addr_unit does not raise the limit, though a larger
unit does make it harder to reach.

Note both in the goal_tuner description.

Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
---
 Documentation/admin-guide/mm/damon/usage.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
index ba47255448564..ea7443b307826 100644
--- a/Documentation/admin-guide/mm/damon/usage.rst
+++ b/Documentation/admin-guide/mm/damon/usage.rst
@@ -455,6 +455,13 @@ the background design of the feature and the name of the selectable algorithms.
 Refer to :ref:`goals directory <sysfs_schemes_quota_goals>` for the goals
 setup.
 
+The ``temporal`` algorithm converts the size quota into basis points, so it
+cannot represent a ``bytes`` value larger than 429496 on 32-bit kernels, or
+1844674407370955 on 64-bit ones.  Larger values are handled as if no size
+quota were set.  :ref:`addr_unit <damon_design_addr_unit>` does not raise the
+limit, but it does make the limit harder to reach: with a larger unit, the
+same amount of memory is written to ``bytes`` as a smaller number.
+
 You can set the action-failed memory quota charging ratio by writing the
 numerator and the denominator for the ratio to ``fail_charge_num`` and
 ``fail_charge_denom`` files, respectively.  Reading those files will return the
-- 
2.53.0

Re: [PATCH v2 3/3] Docs/admin-guide/mm/damon/usage: document the temporal tuner's quota limit

[SJ Park]

On Sun, 20 Sep 2026 11:31:11 +0900 Donggeun Yoo <donggeunyoo.kernel@gmail.com> wrote:

> The temporal goal tuner converts the size quota into basis points in an
> unsigned long, so a quotas/bytes above ULONG_MAX / 10000 does not fit,
> and is handled as if no size quota were set.  On 32-bit that is 429496
> bytes, well below the "echo $((1024*1024*1024)) > quotas/bytes" this
> document instructs.  addr_unit does not raise the limit, though a larger
> unit does make it harder to reach.
> 
> Note both in the goal_tuner description.
> 
> Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
> ---
>  Documentation/admin-guide/mm/damon/usage.rst | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
> index ba47255448564..ea7443b307826 100644
> --- a/Documentation/admin-guide/mm/damon/usage.rst
> +++ b/Documentation/admin-guide/mm/damon/usage.rst
> @@ -455,6 +455,13 @@ the background design of the feature and the name of the selectable algorithms.
>  Refer to :ref:`goals directory <sysfs_schemes_quota_goals>` for the goals
>  setup.
>  
> +The ``temporal`` algorithm converts the size quota into basis points, so it
> +cannot represent a ``bytes`` value larger than 429496 on 32-bit kernels, or
> +1844674407370955 on 64-bit ones.

This feels like too much implementation details.

> Larger values are handled as if no size
> +quota were set.

I don't get what this means.

> :ref:`addr_unit <damon_design_addr_unit>` does not raise the
> +limit, but it does make the limit harder to reach: with a larger unit, the
> +same amount of memory is written to ``bytes`` as a smaller number.

I suggested it might make sense to document this problem, but now I doubt if
this is really helpful for users.  Given the fact that the issue can happen on
only untested setups, I now think this documentation change is better not to be
made.  Correct me if I'm wrong.  If I'm not incorrect, could we drop this
patch?


Thanks,
SJ

[...]

Re: [PATCH v2 3/3] Docs/admin-guide/mm/damon/usage: document the temporal tuner's quota limit

[Donggeun Yoo]

Thank you for the review.

On Sun, 20 Sep 2026 03:49:20 -0700 SJ Park <sj@kernel.org> wrote:
> I now think this documentation change is better not to be made.
> Correct me if I'm wrong.  If I'm not incorrect, could we drop this
> patch?

You are not wrong, and I agree.  Dropped in v3.

Thanks,
Donggeun