[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@xxxxxxxxx>
---
 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