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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>

authored by

Liew Rui Yan and committed by
Andrew Morton 4bdbddb4 bf989ade

+19
+5
Documentation/admin-guide/mm/damon/lru_sort.rst
··· 351 351 # echo 400 > wmarks_mid 352 352 # echo 200 > wmarks_low 353 353 # echo Y > enabled 354 + 355 + Note that this module (damon_lru_sort) cannot run simultaneously with other 356 + DAMON-based special-purpose modules. Refer to :ref:`DAMON design special 357 + purpose modules exclusivity <damon_design_special_purpose_modules_exclusivity>` 358 + for more details.
+5
Documentation/admin-guide/mm/damon/reclaim.rst
··· 318 318 # echo 200 > wmarks_low 319 319 # echo Y > enabled 320 320 321 + Note that this module (damon_reclaim) cannot run simultaneously with other 322 + DAMON-based special-purpose modules. Refer to :ref:`DAMON design special 323 + purpose modules exclusivity <damon_design_special_purpose_modules_exclusivity>` 324 + for more details. 325 + 321 326 .. [1] https://research.google/pubs/pub48551/ 322 327 .. [2] https://lwn.net/Articles/787611/ 323 328 .. [3] https://www.kernel.org/doc/html/latest/mm/free_page_reporting.html
+5
Documentation/admin-guide/mm/damon/stat.rst
··· 45 45 Setting it as ``N`` disables DAMON_STAT. The default value is set by 46 46 ``CONFIG_DAMON_STAT_ENABLED_DEFAULT`` build config option. 47 47 48 + Note that this module (damon_stat) cannot run simultaneously with other 49 + DAMON-based special-purpose modules. Refer to :ref:`DAMON design special 50 + purpose modules exclusivity <damon_design_special_purpose_modules_exclusivity>` 51 + for more details. 52 + 48 53 .. _damon_stat_aggr_interval_us: 49 54 50 55 aggr_interval_us
+4
Documentation/mm/damon/design.rst
··· 853 853 (:doc:`/admin-guide/mm/damon/stat`, :doc:`/admin-guide/mm/damon/reclaim` and 854 854 :doc:`/admin-guide/mm/damon/lru_sort`). 855 855 856 + .. _damon_design_special_purpose_modules_exclusivity: 857 + 858 + Note that these modules currently run in an exclusive manner. If one of those 859 + is already running, others will return ``-EBUSY`` upon start requests. 856 860 857 861 Sample DAMON Modules 858 862 --------------------