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Date: Thu, 13 Feb 2014 11:20:42 -0800
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Subject: Re: [PATCH 00/13] cross rename v4
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: David Howells <dhowells@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
        Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Bruce Fields <bfields@fieldses.org>,
        Christoph Hellwig <hch@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, Zach Brown <zab@redhat.com>,
        Jan Kara <jack@suse.cz>, Andy Lutomirski <luto@amacapital.net>,
        "mszeredi@suse.cz" <mszeredi@suse.cz>
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On Thu, Feb 13, 2014 at 10:56 AM, Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> Fair enough, that allows the thing to be listed, at least.
>
> What about creation?  A new syscall?
>
> Removal?  unlink(2)?
>
> Should stat(2) succeed with a new filetype?

I think it had better work exactly like a special node (eg character
device etc). I don't know about creation (yes, we might even fake it
with mknod(), or just say that the only way to create them is as part
of the union-fs), but removal and renaming should absolutely *not* be
a new system call. That would be a disaster for any system admin,
having to use special tools to edit the filesystem.

Obviously when it is part of a union mount, whiteouts work differently
- they must *not* show up in getdents, and you can't rename/remove a
whiteout anywhere else. But that is obviously part of the union-fs,
nor the low-level filesystem itself.

            Linus