linux-kernel - Re: [RFC] Union mount readdir support in glibc

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Date:	Tue, 11 Mar 2008 18:19:35 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	libc-alpha@...rceware.org, Jan Blunck <jblunck@...e.de>,
	Erez Zadok <ezk@...sunysb.edu>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	Christoph Hellwig <hch@....de>,
	Ulrich Drepper <drepper@...hat.com>,
	Mingming Cao <cmm@...ibm.com>,
	Dave Hansen <haveblue@...ibm.com>
Subject: Re: [RFC] Union mount readdir support in glibc

On Tue, Mar 11, 2008 at 01:09:29AM -0700, Roland McGrath wrote:
> It seems very unlikely you'd come up with a version of this plan that we'd
> find acceptable in glibc.  readdir does buffering, sometimes entry format
> conversion, and it can skip dummy entries.  That's it.  It's not going to
> become a big hairy thing with all kinds of new state.  Sorry.

In the approach we are suggesting, at the minimum, glibc readdir would
have to maintain a unified cache of dirents with the knowlege of
whiteouts (DT_WHT). Would that be too much ?

> 
> This really is the kernel filesystem's problem.  It just doesn't make sense
> to expect userland to implement half of your directory semantics for you.
> What are you going to do when you want to export a union directory to NFS?
> readdir is a filesystem operation.  You're implementing a filesystem.

Not really. In Union Mount, most of the unification support is done at
VFS layer with some support from filesystems (for things like
whiteouts). It is Unionfs which implements a new filesystem to achieve
unification. Unification is not purely a kernel filesystem's problem, it
involves both VFS and FS.

> 
> Exposing DT_WHT entries may be useful as a user feature.  (BSD had unions
> with whiteouts years ago, and their ls et al have options to let you see
> and operate on whiteouts explicitly so users can make sense of strange
> situations with unions.)  But even for that, we'd have to consider the
> compatibility issues.

AFAIK, even BSD implements duplicate elimination and whiteout
suppression in the userland.

Thanks for your comments.

Regards,
Bharata.
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