[PATCH] Docs/mm/damon: document exclusivity of special-purpose modules

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

Add a section in design.rst to explain that DAMON special-purpose kernel
modules (LRU_SORT, RECLAIM, STAT) run in an exclusive manner and return
-EBUSY if another is already running.

Update lru_sort.rst, reclaim.rst and stat.rst by adding cross-references
to this exclusivity rule at the end of their respective Example
sections.

While at it, fix a Sphinx syntax error in the "contents" directive in
design.rst (missing space after :depth:) that prevents the table of
contents from being rendered on docs.kernel.org.

Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
Link: https://lore.kernel.org/damon/20260314002119.79742-1-sj@kernel.org/T/#t
---
 Documentation/admin-guide/mm/damon/lru_sort.rst | 5 +++++
 Documentation/admin-guide/mm/damon/reclaim.rst  | 5 +++++
 Documentation/admin-guide/mm/damon/stat.rst     | 5 +++++
 Documentation/mm/damon/design.rst               | 6 +++++-
 4 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/mm/damon/lru_sort.rst b/Documentation/admin-guide/mm/damon/lru_sort.rst
index 06a46812a728..0198dfe87f4d 100644
--- a/Documentation/admin-guide/mm/damon/lru_sort.rst
+++ b/Documentation/admin-guide/mm/damon/lru_sort.rst
@@ -353,3 +353,8 @@ the LRU-list based page granularity reclamation. ::
     # echo 400 > wmarks_mid
     # echo 200 > wmarks_low
     # echo Y > enabled
+
+Note that this module (damon_lru_sort) cannot run simultaneously with other
+DAMON-based special-purpose modules.  Refer to :ref:`DAMON design special
+purpose modules exclusivity <damon_design_special_purpose_modules_exclusivity>`
+for more details.
diff --git a/Documentation/admin-guide/mm/damon/reclaim.rst b/Documentation/admin-guide/mm/damon/reclaim.rst
index c5b4d8a1b001..a37ce6fdff05 100644
--- a/Documentation/admin-guide/mm/damon/reclaim.rst
+++ b/Documentation/admin-guide/mm/damon/reclaim.rst
@@ -331,6 +331,11 @@ granularity reclamation. ::
     # echo 200 > wmarks_low
     # echo Y > enabled
 
+Note that this module (damon_reclaim) cannot run simultaneously with other
+DAMON-based special-purpose modules.  Refer to :ref:`DAMON design special
+purpose modules exclusivity <damon_design_special_purpose_modules_exclusivity>`
+for more details.
+
 .. [1] https://research.google/pubs/pub48551/
 .. [2] https://lwn.net/Articles/787611/
 .. [3] Documentation/mm/free_page_reporting.rst
diff --git a/Documentation/admin-guide/mm/damon/stat.rst b/Documentation/admin-guide/mm/damon/stat.rst
index 468c122c4259..46c5dd96aa2e 100644
--- a/Documentation/admin-guide/mm/damon/stat.rst
+++ b/Documentation/admin-guide/mm/damon/stat.rst
@@ -45,6 +45,11 @@ You can enable DAMON_STAT by setting the value of this parameter as ``Y``.
 Setting it as ``N`` disables DAMON_STAT.  The default value is set by
 ``CONFIG_DAMON_STAT_ENABLED_DEFAULT`` build config option.
 
+Note that this module (damon_stat) cannot run simultaneously with other
+DAMON-based special-purpose modules.  Refer to :ref:`DAMON design special
+purpose modules exclusivity <damon_design_special_purpose_modules_exclusivity>`
+for more details.
+
 .. _damon_stat_aggr_interval_us:
 
 aggr_interval_us
diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index 0a3b77ec458f..3694c541e65a 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -5,7 +5,7 @@ Design
 ======
 
 .. contents:: :local:
-   :depth:2
+   :depth: 2
 
 .. _damon_design_execution_model_and_data_structures:
 
@@ -960,6 +960,10 @@ more detail, please read the usage documents for those
 (:doc:`/admin-guide/mm/damon/stat`, :doc:`/admin-guide/mm/damon/reclaim` and
 :doc:`/admin-guide/mm/damon/lru_sort`).
 
+.. _damon_design_special_purpose_modules_exclusivity:
+
+Note that these modules currently run in an exclusive manner.  If one of those
+is already running, others will return ``-EBUSY`` upon start requests.
 
 Sample DAMON Modules
 --------------------
-- 
2.53.0