Re: [RFC] Union mount readdir support in glibc

[Jan Blunck <jblunck@suse.de>]

On Thu, Mar 13, Ulrich Drepper wrote:

> There is very little overhead.  Since we copy using getdents multiple
> records it is more efficient than implementing readdir in the kernel.
> This is how efficient normal directory operations must remain.  The only
> slight inefficiency is that we have to copy the entries after getdents()
> because the d_type field is not in the place we expect it at userlevel.
>  For this a new interface could help.

BTW, Since some filesystem always give DT_UNKNOWN an additional stat is
necessary to implement whiteout filtering. I don't want to do that in
kernel-space if possible ...

> Regarding questions you have: if a directory currently is read and file
> are added or removed, all bets are off.
> 
> re seeking: you have to support seeking.  There is no way around it.
> Once again, if any file has been added/removed, all bets are off.  So,
> why not provide a cookie similar to what is done today?  I think it is
> not acceptable to require caching the entire directory content at
> userlevel.  It's bad enough if we have to store the file names for
> duplicate elimination.

Which basically means tracking of the "space" between dirents and maintaining
the relative order of entries. Which is a pain. I already tried to solve this
problem for tmpfs before and it needs a hugh amount of kernel memory for open
directories. In the end I only know of one situation where it is used: very old
glibc when running 32bit applications on 64bit kernel.

Cheers,
Jan