linux-kernel - Re: [rfc git patch] union directory

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Message-Id: <E1L7EUL-0003ej-Su@pomaz-ex.szeredi.hu>
Date:	Mon, 01 Dec 2008 20:31:21 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	bharata@...ux.vnet.ibm.com
CC:	miklos@...redi.hu, linux-fsdevel@...r.kernel.org, jblunck@...e.de,
	dlezcano@...ibm.com, linux-kernel@...r.kernel.org,
	viro@...IV.linux.org.uk
Subject: Re: [rfc git patch] union directory

On Mon, 1 Dec 2008, Bharata B Rao wrote:
> On Mon, Dec 01, 2008 at 10:05:09AM +0530, Bharata B Rao wrote:
> > On Sat, Nov 29, 2008 at 05:33:19PM +0100, Miklos Szeredi wrote:
> > > On Fri, 28 Nov 2008, Bharata B Rao wrote:
> > > 
> > > The plan is to get a simple kernel implementation first which caches
> > > the directory in 'struct file'.
> > 
> > FYI, I did this for a version of Union Mount.
> > (http://lkml.org/lkml/2007/6/20/21) I was maintaining the readdir cache in
> > struct file and the cache was persistant across readdir calls.
> > 
> > Do you have anything different in mind ?

Thanks for the pointer.  Yes something like that.

> 
> The version pointed to by me above had the entire cache stored
> in the struct file of the topmost directory. I guess your plan
> is to cache them per branch and combine them during
> readdir()/getdents() ?

No, the plan is to cache it in the struct file of the topmost
directory.  This should also solve the lseek() issue.

The only concern is that this caching behavior can waste unswappable
kernel memory.  But unless some malicious application actually wants
to exploit this, I don't think it will be a big issue in practice.

Miklos
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