overlayfs: add statfs support

Add support for statfs to the overlayfs filesystem.  As the upper layer
is the target of all write operations assume that the space in that
filesystem is the space in the overlayfs.  There will be some inaccuracy as
overwriting a file will copy it up and consume space we were not expecting,
but it is better than nothing.

Use the upper layer dentry and mount from the overlayfs root inode,
passing the statfs call to that filesystem.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/overlayfs/super.c

@@ -16,17 +16,21 @@
 #include <linux/parser.h>
 #include <linux/module.h>
 #include <linux/sched.h>
+#include <linux/statfs.h>
 #include "overlayfs.h"
 
 MODULE_AUTHOR("Miklos Szeredi <miklos@szeredi.hu>");
 MODULE_DESCRIPTION("Overlay filesystem");
 MODULE_LICENSE("GPL");
 
+#define OVERLAYFS_SUPER_MAGIC 0x794c764f
+
 /* private information held for overlayfs's superblock */
 struct ovl_fs {
 	struct vfsmount *upper_mnt;
 	struct vfsmount *lower_mnt;
 	struct dentry *workdir;
+	long lower_namelen;
 };
 
 struct ovl_dir_cache;
@@ -383,8 +387,35 @@ static void ovl_put_super(struct super_block *sb)
 	kfree(ufs);
 }
 
+/**
+ * ovl_statfs
+ * @sb: The overlayfs super block
+ * @buf: The struct kstatfs to fill in with stats
+ *
+ * Get the filesystem statistics.  As writes always target the upper layer
+ * filesystem pass the statfs to the same filesystem.
+ */
+static int ovl_statfs(struct dentry *dentry, struct kstatfs *buf)
+{
+	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;
+	struct dentry *root_dentry = dentry->d_sb->s_root;
+	struct path path;
+	int err;
+
+	ovl_path_upper(root_dentry, &path);
+
+	err = vfs_statfs(&path, buf);
+	if (!err) {
+		buf->f_namelen = max(buf->f_namelen, ofs->lower_namelen);
+		buf->f_type = OVERLAYFS_SUPER_MAGIC;
+	}
+
+	return err;
+}
+
 static const struct super_operations ovl_super_operations = {
 	.put_super	= ovl_put_super,
+	.statfs		= ovl_statfs,
 };
 
 struct ovl_config {
@@ -556,6 +587,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	struct ovl_entry *oe;
 	struct ovl_fs *ufs;
 	struct ovl_config config;
+	struct kstatfs statfs;
 	int err;
 
 	err = ovl_parse_opt((char *) data, &config);
@@ -617,6 +649,13 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 		goto out_put_workpath;
 	}
 
+	err = vfs_statfs(&lowerpath, &statfs);
+	if (err) {
+		pr_err("overlayfs: statfs failed on lowerpath\n");
+		goto out_put_workpath;
+	}
+	ufs->lower_namelen = statfs.f_namelen;
+
 	ufs->upper_mnt = clone_private_mount(&upperpath);
 	err = PTR_ERR(ufs->upper_mnt);
 	if (IS_ERR(ufs->upper_mnt)) {
@@ -669,6 +708,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 
 	root_dentry->d_fsdata = oe;
 
+	sb->s_magic = OVERLAYFS_SUPER_MAGIC;
 	sb->s_op = &ovl_super_operations;
 	sb->s_root = root_dentry;
 	sb->s_fs_info = ufs;