Docs/mm/damon: document min_nr_regions constraint and rationale

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].

Link: https://lkml.kernel.org/r/20260320052428.213230-1-aethernet65535@gmail.com
Link: https://lore.kernel.org/damon/20260319151528.86490-1-sj@kernel.org/T/#t
Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/admin-guide/mm/damon/lru_sort.rst b/Documentation/admin-guide/mm/damon/lru_sort.rst
index 56690646cf398813d57f75d835f22d20d97b68f0..a7dea7c75a9bbb282b9654f21e08c20117784ff8 100644 (file)
--- 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 442ac5c64795168f3668bc8b0487f2f6c57d8db5..47854c461706cb8ec68f934ebf31400b2d4551f1 100644 (file)
--- 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 838b14d2251932e5b66db031f50dfb48c94488ad..afc7d52bda2f7f5de984e3f9573b3730cbbcf6ad 100644 (file)
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -167,6 +167,13 @@ 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.