[PATCH] cgroup: deprecate remount option changes mount option

[Tejun Heo <tj@kernel.org>]

This patch marks the following features for deprecation.

* Rebinding subsys by remount: Never reached useful state - only works
  on empty hierarchies.

* release_agent update by remount: release_agent itself will be
  replaced with conventional fsnotify notification.

v2: Lennart pointed out that "name=" is necessary for mounts w/o any
    controller attached.  Drop "name=" deprecation.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>
---
 Documentation/feature-removal-schedule.txt |   10 ++++++++++
 kernel/cgroup.c                            |    5 +++++
 2 files changed, 15 insertions(+)

Index: work/Documentation/feature-removal-schedule.txt
===================================================================
--- work.orig/Documentation/feature-removal-schedule.txt
+++ work/Documentation/feature-removal-schedule.txt
@@ -510,3 +510,13 @@ Why:	The pci_scan_bus_parented() interfa
 	convert to using pci_scan_root_bus() so they can supply a list of
 	bus resources when the bus is created.
 Who:	Bjorn Helgaas <bhelgaas@google.com>
+
+----------------------------
+
+What:	cgroup option updates via remount
+When:	March 2013
+Why:	Remount currently allows changing bound subsystems and
+	release_agent.  Rebinding is hardly useful as it only works
+	when the hierarchy is empty and release_agent itself should be
+	replaced with conventional fsnotify.
+Who:	Tejun Heo <tj@kernel.org>
Index: work/kernel/cgroup.c
===================================================================
--- work.orig/kernel/cgroup.c
+++ work/kernel/cgroup.c
@@ -1294,6 +1294,11 @@ static int cgroup_remount(struct super_b
 	if (ret)
 		goto out_unlock;
 
+	/* See feature-removal-schedule.txt */
+	if (opts.subsys_bits != root->actual_subsys_bits || opts.release_agent)
+		pr_warning("cgroup: option changes via remount are deprecated (pid=%d comm=%s)\n",
+			   task_tgid_nr(current), current->comm);
+
 	/* Don't allow flags or name to change at remount */
 	if (opts.flags != root->flags ||
 	    (opts.name && strcmp(opts.name, root->name))) {