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

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Message-ID: <20120817182051.GC29217@decadent.org.uk>
Date:	Fri, 17 Aug 2012 19:20:51 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	viro@...IV.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	apw@...onical.com, nbd@...nwrt.org, neilb@...e.de,
	hramrach@...trum.cz, jordipujolp@...il.com, ezk@....cs.sunysb.edu,
	ricwheeler@...il.com, dhowells@...hat.com, hpj@...la.net,
	sedat.dilek@...glemail.com, penberg@...nel.org,
	goran.cetusic@...il.com, romain@...bokech.com, mszeredi@...e.cz
Subject: Re: [PATCH 05/13] overlayfs: add statfs support

On Wed, Aug 15, 2012 at 05:48:12PM +0200, Miklos Szeredi wrote:
> From: Andy Whitcroft <apw@...onical.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
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