Re: [RFC] Union mounts/writable overlays design

[Brad Boyer <flar@allandria.com>]

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@allandria.com