linux-kernel - Re: [RFC] Union mounts/writable overlays design

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Message-ID: <20091001184935.GA27667@cynthia.pants.nu>
Date:	Thu, 1 Oct 2009 11:49:35 -0700
From:	Brad Boyer <flar@...andria.com>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Jan Blunck <jblunck@...e.de>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Andy Whitcroft <apw@...onical.com>,
	Scott James Remnant <scott@...onical.com>,
	Sandu Popa Marius <sandupopamarius@...il.com>,
	Jan Rekorajski <baggins@...h.mimuw.edu.pl>,
	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	Arnd Bergmann <arnd@...db.de>,
	Vladimir Dronnikov <dronnikov@...il.com>,
	Felix Fietkau <nbd@...nwrt.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] Union mounts/writable overlays design

On Thu, Oct 01, 2009 at 10:55:48AM -0400, Valerie Aurora wrote:
> We need to guarantee that a file system will be read-only for as long
> as it is the bottom layer of a writable overlay.  To do this, we track
> the number of "read-only users" of a file system in its VFS superblock
> structure.  When we mount a writable overlay over a file system, we
> increment its read-only user count.  The file system can only be
> mounted read-write if its read-only users count is zero.
> 
> Todo:
> 
> - Support really really read-only NFS mounts.  See discussion here:
> 
>   http://markmail.org/message/3mkgnvo4pswxd7lp

Is there any way for a file system driver to just come out and say
"I can't guarantee that this mount is really read-only"? I can imagine
this might be an issue for things other than NFS. I think it would be
worthwhile to have a flag maybe on a per sb level that says that even
if it is mounted with the "ro" option that it might not really be stable.
I don't think this is essential, but it would be a good feature as long
as it doesn't damage the design or performance too much.

	Brad Boyer
	flar@...andria.com

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