Subject: Re: [PATCH 01/24] Unionfs: Documentation
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Jan Kara <jack@suse.cz>
Cc: Erez Zadok <ezk@cs.sunysb.edu>, Andrew Morton <akpm@osdl.org>,
       "Josef 'Jeff' Sipek" <jsipek@cs.sunysb.edu>,
       linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
       hch@infradead.org, viro@ftp.linux.org.uk, torvalds@osdl.org,
       mhalcrow@us.ibm.com, David Quigley <dquigley@cs.sunysb.edu>
In-Reply-To: <20070109122644.GB1260@atrey.karlin.mff.cuni.cz>
References: <20070108111852.ee156a90.akpm@osdl.org>
	 <200701082051.l08KpV8b011212@agora.fsl.cs.sunysb.edu>
	 <20070109122644.GB1260@atrey.karlin.mff.cuni.cz>
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Date: Tue, 09 Jan 2007 11:39:38 -0500
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On Tue, 2007-01-09 at 13:26 +0100, Jan Kara wrote:
>   Yes, making fs readonly at VFS level would not work for already opened
> files. But you if you create new union, you could lock down the
> filesystems you are unioning (via s_umount semaphore), go through lists
> of all open fd's on those filesystems and check whether they are open
> for write or not. If some fd is open for writing, you simply fail to
> create the union (and it's upto user to solve the problem). Otherwise
> you mark filesystems as RO and safely proceed with creating the union.
> I guess you must have come up with this solution. So what is the problem
> with it?

Aside from the fact that this is racy (s_umount doesn't protect you
against a process opening a new file while you are busy running through
the open fds to see if you can reset the MS_RDONLY flag) all you will
have achieved is ensure that your client won't write to the file. You
will still be able to Oops.

  Trond

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