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From: Nikita Danilov <nikita@clusterfs.com>
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Date: Mon, 24 Jul 2006 10:45:45 +0400
To: "Joshua Hudson" <joshudson@gmail.com>
Cc: Linux Kernel Mailing List <Linux-Kernel@vger.kernel.org>
Subject: Re: what is necessary for directory hard links
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Joshua Hudson writes:
 > On 7/22/06, Bodo Eggert <7eggert@elstempel.de> wrote:
 > > Horst H. von Brand <vonbrand@inf.utfsm.cl> wrote:
 > >
 > > > Joshua Hudson <joshudson@gmail.com> wrote:
 > > >> This patch is the sum total of all that I had to change in the kernel
 > > >> VFS layer to support hard links to directories
 > > >
 > > > Can't be done, as it creates the possibility of loops.
 > >
 > > Don't do that then?
 > Exactly.
 > 
 > > > Detecting unconnected subgraphs uses a /lot/ of memory; and much worse, you
 > > > have to stop (almost) all filesystem activity while doing it.
 > I know.
 > 
 > I just decided the price is worth it.
 > 
 > In my filesystem, any attempt to create a loop of hard links
 > is detected and cancelled. 

Can you elaborate a bit on this exciting mechanism? Obviously an ability
to efficiently detect loops would be a break-through in a
reference-counted garbage collection, somehow missed for last 40
years. :-)

 >                            Unlinking a directory requires it
 > to be empty if the last link is being removed. "." and ".."
 > links are counted separately from real links, so that is easy.

Nikita.