Docs/mm/damon/design: document allow/reject DAMOS filter behaviors

Update DAMOS filters design document to describe the allow/reject behavior
of filters.

Link: https://lkml.kernel.org/r/20250109175126.57878-8-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 29 insertions, 4 deletions

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index 449eb33688c2..667775bab86c 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -504,9 +504,34 @@ have a list of latency-critical processes.
 
 To let users optimize DAMOS schemes with such special knowledge, DAMOS provides
 a feature called DAMOS filters.  The feature allows users to set an arbitrary
-number of filters for each scheme.  Each filter specifies the type of target
-memory, and whether it should exclude the memory of the type (filter-out), or
-all except the memory of the type (filter-in).
+number of filters for each scheme.  Each filter specifies
+
+- a type of memory (``type``),
+- whether it is for the memory of the type or all except the type
+  (``matching``), and
+- whether it is to allow (include) or reject (exclude) applying
+  the scheme's action to the memory (``allow``).
+
+When multiple filters are installed, each filter is evaluated in the installed
+order.  If a part of memory is matched to one of the filter, next filters are
+ignored.  If the memory passes through the filters evaluation stage because it
+is not matched to any of the filters, applying the scheme's action to it is
+allowed, same to the behavior when no filter exists.
+
+For example, let's assume 1) a filter for allowing anonymous pages and 2)
+another filter for rejecting young pages are installed in the order.  If a page
+of a region that eligible to apply the scheme's action is an anonymous page,
+the scheme's action will be applied to the page regardless of whether it is
+young or not, since it matches with the first allow-filter.  If the page is
+not anonymous but young, the scheme's action will not be applied, since the
+second reject-filter blocks it.  If the page is neither anonymous nor young,
+the page will pass through the filters evaluation stage since there is no
+matching filter, and the action will be applied to the page.
+
+Note that the action can equally be applied to memory that either explicitly
+filter-allowed or filters evaluation stage passed.  It means that installing
+allow-filters at the end of the list makes no practical change but only
+filters-checking overhead.
 
 For efficient handling of filters, some types of filters are handled by the
 core layer, while others are handled by operations set.  In the latter case,
@@ -516,7 +541,7 @@ filter are not counted as the scheme has tried to the region.  In contrast, if
 a memory regions is filtered by an operations set layer-handled filter, it is
 counted as the scheme has tried.  This difference affects the statistics.
 
-Below types of filters are currently supported.
+Below ``type`` of filters are currently supported.
 
 - anonymous page
     - Applied to pages that containing data that not stored in files.