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

[Jan Blunck]

On Tue, May 19, Arnd Bergmann wrote:

> 
> So what would go wrong if you only made them persistent for writable file
> systems, but allowed fallthrough dentries to be discarded for read-only
> file systems? As long as the lower layers don't change, you should still
> be able to reconstruct the same dentries every time you do a readdir, right?
> 

This can work if you do this when the last file descriptor on the directory is
closed. I have a similar patch around for tmpfs readdir (the SLES8 glibc is
seeking on readdir, so it broke some of our build servers that use tmpfs).

One idea would be to separate the "fallthru in tmpfs" handling into its own
library so that it can be shared by filesystems. This would become something
similar to the On-Disk-Format (ODF) approach that Erez Zadok followed with
UnionFS.
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