[RFC PATCH v2.2 15/17] Docs/mm/damon/design: document probe preps

Update DAMON design document for the newly added DAMON probe preps
feature.

Signed-off-by: SJ Park <[email protected]>
---
 Documentation/mm/damon/design.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index c4830bd09498d..7f983e0c39b80 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -309,6 +309,13 @@ Users can therefore know how much of a given DAMON region has a specific data
 attribute by reading the per-region per-probe probe hits counter after each
 aggregation interval.
 
+Users can optionally register probing preparation actions per probe.  If such
+actions are registered, DAMON applies the actions to each region's sampling
+memory before starting the next sampling interval.  Currently only one action,
+``set_pgidle`` is supported.  The action marks the page for the probing target
+memory as access-idle.  This can be useful to be used together with
+``pgidle_unset`` probe filter.
+
 This is a sampling based mechanism.  Hence, it is lightweight but the output
 may include some measurement errors.  The output should be used with good
 understanding of statistics.
-- 
2.47.3