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

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Date:	Wed, 29 Aug 2012 22:54:52 -0700
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
Subject: Re: [PATCH 05/13] overlayfs: add statfs support

On Thu, 2012-08-30 at 00:48 +0200, Miklos Szeredi wrote:
> Ben Hutchings <ben@...adent.org.uk> writes:
> 
> > 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.
> 
> 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.

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

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