[PATCH] Docs/mm/damon: document min_nr_regions constraint and rationale

[Liew Rui Yan <aethernet65535@gmail.com>]

The current DAMON implementation requires 'min_nr_regions' to be at
least 3. However, this constraint is not explicitly documented in the
admin-guide documents, nor is its design rationale explained in the
design document.

Add a section in design.rst to explain the rationale: the virtual
address space monitoring design needs to handle at least three regions
to accommodate two large unmapped areas. While this is specific to
'vaddr', DAMON currently enforces it across all operation sets for
consistency.

Also update reclaim.rst and lru_sort.rst by adding cross-references to
this constraint within their respective 'min_nr_regions' parameter
description sections, ensuring users are aware of the lower bound.

This change is motivated from a recent discussion [1].

[1] https://lore.kernel.org/damon/20260319151528.86490-1-sj@kernel.org/T/#t

Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
---
 Documentation/admin-guide/mm/damon/lru_sort.rst | 4 ++++
 Documentation/admin-guide/mm/damon/reclaim.rst  | 4 ++++
 Documentation/mm/damon/design.rst               | 7 +++++++
 3 files changed, 15 insertions(+)

diff --git a/Documentation/admin-guide/mm/damon/lru_sort.rst b/Documentation/admin-guide/mm/damon/lru_sort.rst
index 20a8378d5a94..3d08baf9802c 100644
--- a/Documentation/admin-guide/mm/damon/lru_sort.rst
+++ b/Documentation/admin-guide/mm/damon/lru_sort.rst
@@ -221,6 +221,10 @@ But, setting this too high could result in increased monitoring overhead.
 Please refer to the DAMON documentation (:doc:`usage`) for more detail.  10 by
 default.
 
+Note that this must be 3 or higher. Please refer to the :ref:`Monitoring
+<damon_design_monitoring>` section of the design document for the rationale
+behind this lower bound.
+
 max_nr_regions
 --------------
 
diff --git a/Documentation/admin-guide/mm/damon/reclaim.rst b/Documentation/admin-guide/mm/damon/reclaim.rst
index 8eba3da8dcee..153bf42645dd 100644
--- a/Documentation/admin-guide/mm/damon/reclaim.rst
+++ b/Documentation/admin-guide/mm/damon/reclaim.rst
@@ -204,6 +204,10 @@ monitoring.  This can be used to set lower-bound of the monitoring quality.
 But, setting this too high could result in increased monitoring overhead.
 Please refer to the DAMON documentation (:doc:`usage`) for more detail.
 
+Note that this must be 3 or higher. Please refer to the :ref:`Monitoring
+<damon_design_monitoring>` section of the design document for the rationale
+behind this lower bound.
+
 max_nr_regions
 --------------
 
diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index dd64f5d7f319..68a25e07203c 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -165,6 +165,13 @@ monitoring attributes, ``sampling interval``, ``aggregation interval``,
 ``update interval``, ``minimum number of regions``, and ``maximum number of
 regions``.
 
+Note that ``minimum number of regions`` must be 3 or higher. This is because the
+virtual address space monitoring is designed to handle at least three regions to
+accommodate two large unmapped areas commonly found in normal virtual address
+spaces. While this restriction might not be strictly necessary for other
+operation sets like ``paddr``, it is currently enforced across all DAMON
+operations for consistency.
+
 To know how user-space can set the attributes via :ref:`DAMON sysfs interface
 <sysfs_interface>`, refer to :ref:`monitoring_attrs <sysfs_monitoring_attrs>`
 part of the documentation.
-- 
2.53.0