Docs/admin-guide/mm/damon: document 'init_regions' feature

This adds description of the 'init_regions' feature in the DAMON usage
document.

Link: https://lkml.kernel.org/r/20211012205711.29216-4-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Amit Shah <amit@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rienjes <rientjes@google.com>
Cc: David Woodhouse <dwmw@amazon.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Leonard Foerster <foersleo@amazon.de>
Cc: Marco Elver <elver@google.com>
Cc: Markus Boehme <markubo@amazon.de>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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

@@ -34,8 +34,9 @@ the reason, this document describes only the debugfs interface
 debugfs Interface
 =================
 
-DAMON exports four files, ``attrs``, ``target_ids``, ``schemes`` and
-``monitor_on`` under its debugfs directory, ``<debugfs>/damon/``.
+DAMON exports five files, ``attrs``, ``target_ids``, ``init_regions``,
+``schemes`` and ``monitor_on`` under its debugfs directory,
+``<debugfs>/damon/``.
 
 
 Attributes
@@ -74,6 +75,42 @@ check it again::
 Note that setting the target ids doesn't start the monitoring.
 
 
+Initial Monitoring Target Regions
+---------------------------------
+
+In case of the debugfs based monitoring, DAMON automatically sets and updates
+the monitoring target regions so that entire memory mappings of target
+processes can be covered.  However, users can want to limit the monitoring
+region to specific address ranges, such as the heap, the stack, or specific
+file-mapped area.  Or, some users can know the initial access pattern of their
+workloads and therefore want to set optimal initial regions for the 'adaptive
+regions adjustment'.
+
+In such cases, users can explicitly set the initial monitoring target regions
+as they want, by writing proper values to the ``init_regions`` file.  Each line
+of the input should represent one region in below form.::
+
+    <target id> <start address> <end address>
+
+The ``target id`` should already in ``target_ids`` file, and the regions should
+be passed in address order.  For example, below commands will set a couple of
+address ranges, ``1-100`` and ``100-200`` as the initial monitoring target
+region of process 42, and another couple of address ranges, ``20-40`` and
+``50-100`` as that of process 4242.::
+
+    # cd <debugfs>/damon
+    # echo "42   1       100
+            42   100     200
+            4242 20      40
+            4242 50      100" > init_regions
+
+Note that this sets the initial monitoring target regions only.  In case of
+virtual memory monitoring, DAMON will automatically updates the boundary of the
+regions after one ``regions update interval``.  Therefore, users should set the
+``regions update interval`` large enough in this case, if they don't want the
+update.
+
+
 Schemes
 -------