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

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Message-ID: <20106.1392309770@warthog.procyon.org.uk>
Date:	Thu, 13 Feb 2014 16:42:50 +0000
From:	David Howells <dhowells@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	dhowells@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, bfields@...ldses.org,
	hch@...radead.org, akpm@...ux-foundation.org, zab@...hat.com,
	jack@...e.cz, luto@...capital.net, mszeredi@...e.cz
Subject: Re: [PATCH 00/13] cross rename v4

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().

Fallthroughs are far more 'interesting'.  Maybe they should appear in
getdents() with a dentry type saying what they are, but give you EREMOTE or
something if you try to follow them.

Note that there is space in d_flags & DCACHE_ENTRY_TYPE for a whiteout type.
I would, however, mark fallthroughs by a separate flag.  So that the union
dentry will mirror the source dentry's type.

> - Can the user remove them?

Overwriting whiteouts and fallthroughs and unlinking fallthroughs I don't see
as a problem where they can be treated as normal negative dentries and normal
files in this regard.

However, what do you do about non-opaque directories that may or may not have
been unioned if you try and follow a dirent that would be a subdirectory that
hasn't been copied up?

David
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