Docs/admin-guide/mm/damon/usage: document DAMOS filters of sysfs

Document about the newly added files for DAMOS filters on the DAMON usage
document.

Link: https://lkml.kernel.org/r/20221205230830.144349-11-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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

@@ -87,6 +87,8 @@ comma (","). ::
     │ │ │ │ │ │ │ quotas/ms,bytes,reset_interval_ms
     │ │ │ │ │ │ │ │ weights/sz_permil,nr_accesses_permil,age_permil
     │ │ │ │ │ │ │ watermarks/metric,interval_us,high,mid,low
+    │ │ │ │ │ │ │ filters/nr_filters
+    │ │ │ │ │ │ │ │ 0/type,matching,memcg_id
     │ │ │ │ │ │ │ stats/nr_tried,sz_tried,nr_applied,sz_applied,qt_exceeds
     │ │ │ │ │ │ │ tried_regions/
     │ │ │ │ │ │ │ │ 0/start,end,nr_accesses,age
@@ -151,6 +153,8 @@ number (``N``) to the file creates the number of child directories named as
 moment, only one context per kdamond is supported, so only ``0`` or ``1`` can
 be written to the file.
 
+.. _sysfs_contexts:
+
 contexts/<N>/
 -------------
 
@@ -268,8 +272,8 @@ schemes/<N>/
 ------------
 
 In each scheme directory, five directories (``access_pattern``, ``quotas``,
-``watermarks``, ``stats``, and ``tried_regions``) and one file (``action``)
+``watermarks``, ``filters``, ``stats``, and ``tried_regions``) and one file
-exist.
+(``action``) exist.
 
 The ``action`` file is for setting and getting what action you want to apply to
 memory regions having specific access pattern of the interest.  The keywords
@@ -347,6 +351,46 @@ as below.
 
 The ``interval`` should written in microseconds unit.
 
+schemes/<N>/filters/
+--------------------
+
+Users could know something more than the kernel for specific types of memory.
+In the case, users could do their own management for the memory and hence
+doesn't want DAMOS bothers that.  Users could limit DAMOS by setting the access
+pattern of the scheme and/or the monitoring regions for the purpose, but that
+can be inefficient in some cases.  In such cases, users could set non-access
+pattern driven filters using files in this directory.
+
+In the beginning, this directory has only one file, ``nr_filters``.  Writing a
+number (``N``) to the file creates the number of child directories named ``0``
+to ``N-1``.  Each directory represents each filter.  The filters are evaluated
+in the numeric order.
+
+Each filter directory contains three files, namely ``type``, ``matcing``, and
+``memcg_path``.  You can write one of two special keywords, ``anon`` for
+anonymous pages, or ``memcg`` for specific memory cgroup filtering.  In case of
+the memory cgroup filtering, you can specify the memory cgroup of the interest
+by writing the path of the memory cgroup from the cgroups mount point to
+``memcg_path`` file.  You can write ``Y`` or ``N`` to ``matching`` file to
+filter out pages that does or does not match to the type, respectively.  Then,
+the scheme's action will not be applied to the pages that specified to be
+filtered out.
+
+For example, below restricts a DAMOS action to be applied to only non-anonymous
+pages of all memory cgroups except ``/having_care_already``.::
+
+    # echo 2 > nr_filters
+    # # filter out anonymous pages
+    echo anon > 0/type
+    echo Y > 0/matching
+    # # further filter out all cgroups except one at '/having_care_already'
+    echo memcg > 1/type
+    echo /having_care_already > 1/memcg_path
+    echo N > 1/matching
+
+Note that filters could be ignored depend on the running DAMON operations set
+`implementation <sysfs_contexts>`.
+
 .. _sysfs_schemes_stats:
 
 schemes/<N>/stats/