[PATCH v2 1/2] cgroup: Add a hint to allow v1 controller to use v2 behavior

[Waiman Long <longman@redhat.com>]

For some cgroup controllers, the behavior on cgroup v1 differs somewhat
from cgroup v2. To indicate that a controller can use v2 behavior
in cgroup v1, a new cgroupfs mount option "v2_mode" is added. This
is essentially a hint, there is no obligation for a controller to
comply with this semantics.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/cgroup-defs.h | 5 +++++
 kernel/cgroup/cgroup-v1.c   | 6 ++++++
 2 files changed, 11 insertions(+)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 09f4c7d..fae33ed 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -74,6 +74,11 @@ enum {
 	 * aren't writeable from inside the namespace.
 	 */
 	CGRP_ROOT_NS_DELEGATE	= (1 << 3),
+
+	/*
+	 * A hint that v1 cgroup should use v2 behavior, if applicable.
+	 */
+	CGRP_ROOT_V2_MODE	= (1 << 4),
 };
 
 /* cftype->flags */
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 7bf4b15..77e22a4 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -846,6 +846,8 @@ static int cgroup1_show_options(struct seq_file *seq, struct kernfs_root *kf_roo
 		seq_puts(seq, ",noprefix");
 	if (root->flags & CGRP_ROOT_XATTR)
 		seq_puts(seq, ",xattr");
+	if (root->flags & CGRP_ROOT_V2_MODE)
+		seq_puts(seq, ",v2_mode");
 
 	spin_lock(&release_agent_path_lock);
 	if (strlen(root->release_agent_path))
@@ -900,6 +902,10 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
 			opts->cpuset_clone_children = true;
 			continue;
 		}
+		if (!strcmp(token, "v2_mode")) {
+			opts->flags |= CGRP_ROOT_V2_MODE;
+			continue;
+		}
 		if (!strcmp(token, "xattr")) {
 			opts->flags |= CGRP_ROOT_XATTR;
 			continue;
-- 
1.8.3.1