Message-ID: <1346306092.17040.3.camel@deadeye.wl.decadent.org.uk>
Subject: Re: [PATCH 05/13] overlayfs: add statfs support
From: Ben Hutchings <ben@decadent.org.uk>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: viro@ZenIV.linux.org.uk, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, hch@infradead.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org,
        apw@canonical.com, nbd@openwrt.org, neilb@suse.de, hramrach@centrum.cz,
        jordipujolp@gmail.com, ezk@fsl.cs.sunysb.edu, ricwheeler@gmail.com,
        dhowells@redhat.com, hpj@urpla.net, sedat.dilek@googlemail.com,
        penberg@kernel.org, goran.cetusic@gmail.com, romain@orebokech.com
Date: Wed, 29 Aug 2012 22:54:52 -0700
In-Reply-To: <87d32971s3.fsf@tucsk.pomaz.szeredi.hu>
References: <1345045700-9062-1-git-send-email-miklos@szeredi.hu>
	 <1345045700-9062-6-git-send-email-miklos@szeredi.hu>
	 <20120817182051.GC29217@decadent.org.uk>
	 <87d32971s3.fsf@tucsk.pomaz.szeredi.hu>
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On Thu, 2012-08-30 at 00:48 +0200, Miklos Szeredi wrote:
> Ben Hutchings <ben@decadent.org.uk> writes:
>=20
> > On Wed, Aug 15, 2012 at 05:48:12PM +0200, Miklos Szeredi wrote:
> >> From: Andy Whitcroft <apw@canonical.com>
> >>=20
> >> Add support for statfs to the overlayfs filesystem.  As the upper laye=
r
> >> is the target of all write operations assume that the space in that
> >> filesystem is the space in the overlayfs.  There will be some inaccura=
cy as
> >> overwriting a file will copy it up and consume space we were not expec=
ting,
> >> but it is better than nothing.
> >>=20
> >> Use the upper layer dentry and mount from the overlayfs root inode,
> >> passing the statfs call to that filesystem.
> > [...]=20
> >> +/**
> >> + * 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 =3D 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.
>=20
> Maybe.  I've never seen any app use f_namelen for anything useful.
[...]

If I'm not mistaken, glibc uses it to implement pathconf(_PC_NAME_MAX),
which may be used by applications in conjunction with readdir_r().

Ben.

--=20
Ben Hutchings
Quantity is no substitute for quality, but it's the only one we've got.

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