cpuset: Use fs_context

Make the cpuset filesystem use the filesystem context.  This is potentially
tricky as the cpuset fs is almost an alias for the cgroup filesystem, but
with some special parameters.

This can, however, be handled by setting up an appropriate cgroup
filesystem and returning the root directory of that as the root dir of this
one.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/cgroup/cpuset.c

@@ -39,6 +39,7 @@
 #include <linux/memory.h>
 #include <linux/export.h>
 #include <linux/mount.h>
+#include <linux/fs_context.h>
 #include <linux/namei.h>
 #include <linux/pagemap.h>
 #include <linux/proc_fs.h>
@@ -372,25 +373,52 @@ static inline bool is_in_v2_mode(void)
  * users. If someone tries to mount the "cpuset" filesystem, we
  * silently switch it to mount "cgroup" instead
  */
-static struct dentry *cpuset_mount(struct file_system_type *fs_type,
-			 int flags, const char *unused_dev_name, void *data)
-{
-	struct file_system_type *cgroup_fs = get_fs_type("cgroup");
-	struct dentry *ret = ERR_PTR(-ENODEV);
-	if (cgroup_fs) {
-		char mountopts[] =
-			"cpuset,noprefix,"
-			"release_agent=/sbin/cpuset_release_agent";
-		ret = cgroup_fs->mount(cgroup_fs, flags,
-					   unused_dev_name, mountopts);
-		put_filesystem(cgroup_fs);
+static int cpuset_get_tree(struct fs_context *fc)
+{
+	struct file_system_type *cgroup_fs;
+	struct fs_context *new_fc;
+	int ret;
+
+	cgroup_fs = get_fs_type("cgroup");
+	if (!cgroup_fs)
+		return -ENODEV;
+
+	new_fc = fs_context_for_mount(cgroup_fs, fc->sb_flags);
+	if (IS_ERR(new_fc)) {
+		ret = PTR_ERR(new_fc);
+	} else {
+		static const char agent_path[] = "/sbin/cpuset_release_agent";
+		ret = vfs_parse_fs_string(new_fc, "cpuset", NULL, 0);
+		if (!ret)
+			ret = vfs_parse_fs_string(new_fc, "noprefix", NULL, 0);
+		if (!ret)
+			ret = vfs_parse_fs_string(new_fc, "release_agent",
+					agent_path, sizeof(agent_path) - 1);
+		if (!ret)
+			ret = vfs_get_tree(new_fc);
+		if (!ret) {	/* steal the result */
+			fc->root = new_fc->root;
+			new_fc->root = NULL;
+		}
+		put_fs_context(new_fc);
 	}
+	put_filesystem(cgroup_fs);
 	return ret;
 }
 
+static const struct fs_context_operations cpuset_fs_context_ops = {
+	.get_tree	= cpuset_get_tree,
+};
+
+static int cpuset_init_fs_context(struct fs_context *fc)
+{
+	fc->ops = &cpuset_fs_context_ops;
+	return 0;
+}
+
 static struct file_system_type cpuset_fs_type = {
 	.name			= "cpuset",
-	.mount = cpuset_mount,
+	.init_fs_context	= cpuset_init_fs_context,
 };
 
 /*