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

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Message-ID: <20080314150638.GA27730@T61>
Date:	Fri, 14 Mar 2008 16:07:12 +0100
From:	Jan Blunck <jblunck@...e.de>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	bharata@...ux.vnet.ibm.com, libc-alpha@...rceware.org,
	Erez Zadok <ezk@...sunysb.edu>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	Christoph Hellwig <hch@....de>, Mingming Cao <cmm@...ibm.com>,
	Dave Hansen <haveblue@...ibm.com>
Subject: Re: [RFC] Union mount readdir support in glibc

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
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