selftests/damon/sysfs.py: validate memcg_path staging readback

Add a dedicated test at the end of main() that stages memcg_path via sysfs
and verifies its readback.  Configure the memcg filter before start(), do
not call commit(), and ignore start() failures so the test does not depend
on CONFIG_MEMCG or cgroup layout.  Call stop() for cleanup without
checking its return value.

Link: https://lore.kernel.org/D2B37130D38E09AC+20260601090634.241864-1-niecheng1@uniontech.com
Link: https://lore.kernel.org/20260629144812.134159-3-sj@kernel.org
Signed-off-by: Cheng Nie <niecheng1@uniontech.com>
Signed-off-by: SJ Park <sj@kernel.org>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/sysfs.py

@@ -250,6 +250,35 @@ 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(
+                        ops_filters=[memcg_filter])],
+                    )])])
+    kdamonds.start()
+
+    shown, rd_err = _damon_sysfs.read_file(
+            os.path.join(memcg_filter.sysfs_dir(), 'memcg_path'))
+    if rd_err is not None:
+        print('memcg_path staging: sysfs read (%s)' % rd_err)
+        kdamonds.stop()
+        exit(1)
+    if shown.rstrip('\n') != memcg_filter.memcg_path:
+        print('memcg_path staging: memcg_path readback '
+              '(shown=%s, expected=%s)' %
+              (shown.rstrip('\n'), memcg_filter.memcg_path))
+        kdamonds.stop()
+        exit(1)
+
+    kdamonds.stop()
+    kdamonds = None
+
 def main():
     global kdamonds
     kdamonds = _damon_sysfs.Kdamonds(
@@ -356,5 +385,7 @@ def main():
     assert_ctxs_committed(kdamonds)
     kdamonds.stop()
 
+    test_memcg_filter_memcg_path_staging()
+
 if __name__ == '__main__':
     main()