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

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.

This change is motivated from another discussion [1].

Link: https://lkml.kernel.org/r/20260315162945.80994-1-sj@kernel.org
Link: https://lore.kernel.org/damon/20260314002119.79742-1-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: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/damon/lru_sort.rst

@@ -351,3 +351,8 @@ 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.

Documentation/admin-guide/mm/damon/reclaim.rst

@@ -318,6 +318,11 @@ 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] https://www.kernel.org/doc/html/latest/mm/free_page_reporting.html

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

Documentation/mm/damon/design.rst

@@ -853,6 +853,10 @@ 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
 --------------------