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

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Date:	Fri, 14 Mar 2008 00:13:00 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	bharata@...ux.vnet.ibm.com, libc-alpha@...rceware.org,
	Jan Blunck <jblunck@...e.de>, Erez Zadok <ezk@...sunysb.edu>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Christoph Hellwig <hch@....de>, Mingming Cao <cmm@...ibm.com>,
	Dave Hansen <haveblue@...ibm.com>
Subject: Re: [RFC] Union mount readdir support in glibc

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Al Viro wrote:
> How about "the first entry returned by getdents(3) after open() is a whiteout
> for e.g. '.'"?  No fstat needed, zero impact for normal directories,
> zero impact for any binaries on old kernels (where you wouldn't have
> unions) and zero impact for old binaries on new kernels unless they
> do getdents() on directory that happens to be a union.

Your definition of "zero impact" doesn't quite match mine.  This would
require significant changes.


> And no lockstep...

Of course there is lockstep.  It is not under the application's control
whether a directory is a union fs or not.  Every implementation except a
pure kernel implementation has this problem.


>> - - How does this work with NFS?
> 
> It won't, kernel-side or done in userland.

Why wouldn't a kernel-side implementation work?


> Actually, do we really need it other than to 0 and to current position
> (i.e. full rewind and a no-op)?

Ever heard of the little function "telldir"?

- --
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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