Docs/admin-guide/mm/damon/usage: document empty target regions commit behavior

Committing a monitoring target with empty target regions is for keeping
the current monitoring results.  This behavior was introduced by commit
973233600676 ("mm/damon/sysfs: update monitoring target regions for online
input commit").  The behavior is not documented, though.  Update the usage
document for clarifying this behavior.

Link: https://lkml.kernel.org/r/20251026182216.118200-5-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
index 98958975604d941ebcfb2b82b34721f6247675ac..c630f2662695d50ebd1c95513c188e975dbd6bda 100644 (file)
--- a/Documentation/admin-guide/mm/damon/usage.rst
+++ b/Documentation/admin-guide/mm/damon/usage.rst
@@ -134,7 +134,8 @@ Users can write below commands for the kdamond to the ``state`` file.
 - ``on``: Start running.
 - ``off``: Stop running.
 - ``commit``: Read the user inputs in the sysfs files except ``state`` file
-  again.
+  again.  Monitoring :ref:`target region <sysfs_regions>` inputs are also be
+  ignored if no target region is specified.
 - ``update_tuned_intervals``: Update the contents of ``sample_us`` and
   ``aggr_us`` files of the kdamond with the auto-tuning applied ``sampling
   interval`` and ``aggregation interval`` for the files.  Please refer to
@@ -289,6 +290,11 @@ In the beginning, this directory has only one file, ``nr_regions``.  Writing a
 number (``N``) to the file creates the number of child directories named ``0``
 to ``N-1``.  Each directory represents each initial monitoring target region.
 
+If ``nr_regions`` is zero when committing new DAMON parameters online (writing
+``commit`` to ``state`` file of :ref:`kdamond <sysfs_kdamond>`), the commit
+logic ignores the target regions.  In other words, the current monitoring
+results for the target are preserved.
+
 .. _sysfs_region:
 
 regions/<N>/