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

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Message-Id: <20080311080929.B076D26F991@magilla.localdomain>
Date:	Tue, 11 Mar 2008 01:09:29 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	bharata@...ux.vnet.ibm.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

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.

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.

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.

Thanks,
Roland
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