[Devel] Re: [RFC] Control Groups Roadmap ideas

[Li Zefan]

Paul Menage wrote:
> 3) Subsystem dependencies
> -----
> 
> This would be a fairly simple change, essentially allowing one
> subsystem to require that it only be mounted on a hierarchy when some
> other subsystem was also present. The implementation would probably be
> a callback that allows a subsystem to confirm whether it's prepared to
> be included in a proposed hierarchy containing a specified subsystem
> bitmask; it would be able to prevent the hierarchy from being created
> by giving an error return. An example of a use for this would be a
> swap subsystem that is mostly independent of the memory controller,
> but uses the page-ownership tracking of the memory controller to
> determine which cgroup to charge swap pages to. Hence it would require
> that it only be mounted on a hierarchy that also included a memory
> controller. The memory controller would make no such requirement by
> itself, so could be used on its own without the swap controller.
> 

Sounds good, and I wrote a prototype in a quick:

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index a6a6035..091bc21 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -254,6 +254,7 @@ struct cgroup_subsys {
 			struct cgroup *cgrp);
 	void (*post_clone)(struct cgroup_subsys *ss, struct cgroup *cgrp);
 	void (*bind)(struct cgroup_subsys *ss, struct cgroup *root);
+	int (*can_mount)(struct cgroup_subsys *ss, unsigned long subsys_bits);
 	int subsys_id;
 	int active;
 	int disabled;
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 62f1a52..3d43ff2 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -824,6 +824,25 @@ static int parse_cgroupfs_options(char *data,
 	return 0;
 }
 
+static int check_mount(unsigned long subsys_bits)
+{
+	int i;
+	int ret;
+	struct cgroup_subsys *ss;
+
+	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+		ss = subsys[i];
+
+		if (test_bit(i, &subsys_bits) && ss->can_mount) {
+			ret = ss->can_mount(ss, subsys_bits);
+			if (ret)
+				return ret;
+		}
+	}
+
+	return 0;
+}
+
 static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 {
 	int ret = 0;
@@ -839,6 +858,10 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 	if (ret)
 		goto out_unlock;
 
+	ret = check_mount(opts.subsys_bits);
+	if (ret)
+		goto out_unlock;
+
 	/* Don't allow flags to change at remount */
 	if (opts.flags != root->flags) {
 		ret = -EINVAL;
@@ -959,6 +982,13 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
 		return ret;
 	}
 
+	ret = check_mount(opts.subsys_bits);
+	if (ret) {
+		if (opts.release_agent)
+			kfree(opts.release_agent);
+		return ret;
+	}
+
 	root = kzalloc(sizeof(*root), GFP_KERNEL);
 	if (!root) {
 		if (opts.release_agent)
-------

for the example about swap controller and memory controller:

static int swap_cgroup_can_mount(struct cgroup_subsys *ss,
				unsigned long subsys_bits)
{
	if (!test_bit(mem_cgroup_subsys_id, &subsys_bits))
		return -EINVAL;
        return 0;
}

'mem_cgroup_subsys_id' is a member of enum cgroup_subsys_id defined in cgroup.h
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