Add a dedicated test at the end of main() that writes memcg_path to
sysfs and checks the readback path.  Validate the readback rather than
a derived memcg_id so the test stays focused on DAMON sysfs behavior
and avoids depending on the local userspace cgroup mount layout.

Signed-off-by: Cheng Nie <niecheng1@uniontech.com>
---
 tools/testing/selftests/damon/sysfs.py | 39 ++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
index aa03a1187489..0f20bc9c46b7 100755
--- a/tools/testing/selftests/damon/sysfs.py
+++ b/tools/testing/selftests/damon/sysfs.py
@@ -250,6 +250,43 @@ def assert_ctxs_committed(kdamonds):
             if ctx in ctxs_paused_for_dump:
                 ctx.pause = False
 
+def test_memcg_filter_memcg_path_staging():
+    global kdamonds
+    memcg_filter = _damon_sysfs.DamosFilter(
+            type_='memcg', matching=True, allow=True, memcg_path='/')
+    kdamonds = _damon_sysfs.Kdamonds(
+            [_damon_sysfs.Kdamond(
+                contexts=[_damon_sysfs.DamonCtx(
+                    targets=[_damon_sysfs.DamonTarget(pid=-1)],
+                    schemes=[_damon_sysfs.Damos()],
+                    )])])
+    err = kdamonds.start()
+    if err is not None:
+        fail('memcg_path staging: kdamond start', {'error': err})
+
+    context = _damon_sysfs.DamonCtx(
+            targets=[_damon_sysfs.DamonTarget(pid=-1)],
+            schemes=[_damon_sysfs.Damos(ops_filters=[memcg_filter])])
+    context.idx = 0
+    context.kdamond = kdamonds.kdamonds[0]
+    kdamonds.kdamonds[0].contexts = [context]
+
+    err = kdamonds.kdamonds[0].commit()
+    if err is not None:
+        fail('memcg_path staging: kdamond commit', {'error': err})
+
+    shown, rd_err = _damon_sysfs.read_file(
+            os.path.join(memcg_filter.sysfs_dir(), 'memcg_path'))
+    if rd_err is not None:
+        fail('memcg_path staging: sysfs read', {'error': rd_err})
+    assert_true(shown.rstrip('\n') == memcg_filter.memcg_path,
+                'memcg_path readback', {'shown': shown})
+
+    err = kdamonds.stop()
+    if err is not None:
+        fail('memcg_path staging: kdamond stop', {'error': err})
+    kdamonds = None
+
 def main():
     global kdamonds
     kdamonds = _damon_sysfs.Kdamonds(
@@ -356,5 +393,7 @@ def main():
     assert_ctxs_committed(kdamonds)
     kdamonds.stop()
 
+    test_memcg_filter_memcg_path_staging()
+
 if __name__ == '__main__':
     main()
-- 
2.51.0