Re: [RFC][PATCH 10/14] In-kernel file copy between union mounted filesystems

Bharata B Rao
2007-05-23T10:34:22Z

On Tue, May 22, 2007 at 08:35:17AM -0400, Shaya Potter wrote:
> Bharata B Rao wrote:
> >
> >In case of regular files, when we copyup a file, we are actually preventing
> >any writes to the lower layers (which we have designated as read only).
> >
> >Applying the same logic to devices, what do we achieve by copying them up ?
> >How does it matter if we write to the device through a node in the upper
> >layer or in the lower layer. Both the writes eventually do the same thing.
>
> What happens if the lower layer is on a read only medium. But the top
> layer is RW. Why can't one change permissions? In your model, one can't.
>
> What happens if one wants to share a lower layer read-only (I'm doing
> this with my research into uses of union file systems), one doesn't want
> permission change in one use of the lower layer to affect any of the
> other uses.

Ok, makes sense. Thanks for the clarification.

So looks like in addition to copyup on open (which is what we do currently)
there is a case for doing copyups for other situations also.

Regards,
Bharata.