linux-kernel - Re: [PATCH 00/32] VFS based Union Mount (V3)

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Message-Id: <E1M6Mez-0000tG-3q@pomaz-ex.szeredi.hu>
Date:	Tue, 19 May 2009 12:35:01 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	jblunck@...e.de
CC:	miklos@...redi.hu, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	bharata@...ibm.com, dwmw2@...radead.org, mszeredi@...e.cz,
	vaurora@...hat.com
Subject: Re: [PATCH 00/32] VFS based Union Mount (V3)

On Tue, 19 May 2009, Jan Blunck wrote:
> The directory in the topmost filesystem is created during
> lookup. The contents of the directory isn't copied up presistently
> at that point in time. Therefore you have an empty directory in the
> topmost filesystem after the lookup. This was necessary to get rid
> of the union_relookup_topmost() calls during create, mknod, mkdir
> etc.
> 
> When readdir is called, the topmost directory is filed up with
> fallthru entries which are persistently stored. This is only
> necessary to get readdir right wrt POSIX. During lookup the fallthru
> dentry, which is in fact a special negative dentry, is ignored and
> therefore the lookup continues on the lower filesystem.

So this means that the topmost branch always needs to be writable,
right?  It isn't possible to make a union of two iso9660 filesystems,
for example?

Thanks,
Miklos
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