linux-kernel - Re: [PATCH 00/13] cross rename v4

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Date:	Thu, 13 Feb 2014 10:21:42 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	David Howells <dhowells@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Bruce Fields <bfields@...ldses.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zach Brown <zab@...hat.com>, Jan Kara <jack@...e.cz>,
	"mszeredi@...e.cz" <mszeredi@...e.cz>
Subject: Re: [PATCH 00/13] cross rename v4

On Thu, Feb 13, 2014 at 9:28 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Thu, Feb 13, 2014 at 5:42 PM, David Howells <dhowells@...hat.com> wrote:
>> Miklos Szeredi <miklos@...redi.hu> wrote:
>>
>>> Regarding whiteouts, I raised a couple of questions that nobody answered
>>> yet, so let me ask again.
>>>
>>> - If a filesystem containing whiteouts (fallthroughs, etc...) is mounted as
>>>   not part of a union, how are these special entities represented to
>>>   userspace?
>>
>> I would suggest that whiteouts appear as otherwise negative dentries and that
>> they don't appear in getdents().
>
> I'd argue that this is an administration nightmare.  E.g. what if the
> a backup needs to be made of the rw layer?
>
> Will rmdir work normally in a directory containing whiteouts?  Will
> the VFS take care of that, just like if it was part of a union?  Or
> will it fail with ENOTEMPTY despite *appearing* empty?
>
> And zillion other problems related to the fact that things happen to a
> filesystem even when they do not appear to happen ("mv foo bar; mv bar
> foo" has side effects).

Are there any users of unions / overlays who will want to modify the
bottom layer after creating the top layer?  I'm starting to think that
changing the bottom layer should require userspace to do a three-way
merge or something and explicitly decide what it wants to do.

--Andy
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