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

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Date:	Thu, 30 Aug 2012 00:48:28 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Ben Hutchings <ben@...adent.org.uk>
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

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.

>
> 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.)

Yeah we could do that.

Thanks,
Miklos
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