cgroup: Add deprecation message to legacy freezer controller

As explained in the commit 76f969e ("cgroup: cgroup v2 freezer"),
the original freezer is imperfect, some users may unwittingly rely on it
when there exists the alternative of v2. Print a message when it happens
and explain that in the docs.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/admin-guide/cgroup-v1/freezer-subsystem.rst

@@ -125,3 +125,7 @@ to unfreeze all tasks in the container::
 
 This is the basic mechanism which should do the right thing for user space task
 in a simple scenario.
+
+This freezer implementation is affected by shortcomings (see commit
+76f969e8948d8 ("cgroup: cgroup v2 freezer")) and cgroup v2 freezer is
+recommended.

kernel/cgroup/legacy_freezer.c

@@ -430,9 +430,11 @@ static ssize_t freezer_write(struct kernfs_open_file *of,
 
 	if (strcmp(buf, freezer_state_strs(0)) == 0)
 		freeze = false;
-	else if (strcmp(buf, freezer_state_strs(CGROUP_FROZEN)) == 0)
+	else if (strcmp(buf, freezer_state_strs(CGROUP_FROZEN)) == 0) {
+		pr_info_once("Freezing with imperfect legacy cgroup freezer. "
+			     "See cgroup.freeze of cgroup v2\n");
 		freeze = true;
-	else
+	} else
 		return -EINVAL;
 
 	freezer_change_state(css_freezer(of_css(of)), freeze);