Re: [PATCH 05/13] overlayfs: add statfs support

Ben Hutchings
2012-08-17T18:21:08Z

On Wed, Aug 15, 2012 at 05:48:12PM +0200, Miklos Szeredi wrote:
> From: Andy Whitcroft <apw@canonical.com>
>
> 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.
[...]
> +/**
> + * 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 dentry *root_dentry = dentry->d_sb->s_root;
> + struct path path;
> + ovl_path_upper(root_dentry, &path);
> +
> + if (!path.dentry->d_sb->s_op->statfs)
> + return -ENOSYS;
> + return path.dentry->d_sb->s_op->statfs(path.dentry, buf);
> +}
[...]

In case f_namelen differs between the upper and lower filesystems, you
need to return the greater of the two.

Should f_type be overridden to indicate overlayfs? I'm not sure what
userland is likely to do with f_type. (For presentation to the user,
it should get the mount type name with getmntent() or libmount. And
that will just work.)

Ben.

--
Ben Hutchings
We get into the habit of living before acquiring the habit of thinking.
- Albert Camus