cgroup: warn if "xattr" is specified with "sane_behavior"

Mount option "xattr" is no longer necessary as it's enabled by default
on kernfs.  Warn if "xattr" is specified with "sane_behavior" so that
the option can be removed in the future.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -264,6 +264,8 @@ enum {
 	 * - "release_agent" and "notify_on_release" are removed.
 	 *   Replacement notification mechanism will be implemented.
 	 *
+	 * - "xattr" mount option is deprecated.  kernfs always enables it.
+	 *
 	 * - cpuset: tasks will be kept in empty cpusets when hotplug happens
 	 *   and take masks of ancestors with non-empty cpus/mems, instead of
 	 *   being moved to an ancestor.

kernel/cgroup.c

@@ -1267,6 +1267,9 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
 			pr_err("cgroup: sane_behavior: clone_children is not allowed\n");
 			return -EINVAL;
 		}
+
+		if (opts->flags & CGRP_ROOT_XATTR)
+			pr_warning("cgroup: sane_behavior: xattr is always available, flag unnecessary\n");
 	}
 
 	/*