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 [1]
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>

2 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/admin-guide/mm/damon/lru_sort.rst b/Documentation/admin-guide/mm/damon/lru_sort.rst
index 56690646cf39..a7dea7c75a9b 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 442ac5c64795..47854c461706 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
 --------------