linux-kernel - Re: what is necessary for directory hard links

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Message-Id: <200607240725.k6O7PTp1012347@turing-police.cc.vt.edu>
Date:	Mon, 24 Jul 2006 03:25:29 -0400
From:	Valdis.Kletnieks@...edu
To:	Nikita Danilov <nikita@...sterfs.com>
Cc:	Joshua Hudson <joshudson@...il.com>,
	Linux Kernel Mailing List <Linux-Kernel@...r.Kernel.ORG>
Subject: Re: what is necessary for directory hard links

On Mon, 24 Jul 2006 10:45:45 +0400, Nikita Danilov said:
> Joshua Hudson writes:

>  > 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

It's actually pretty trivial to do if it's a toy filesystem and all the
relevant inodes are in-memory already.  The hard-to-solve part is getting
around the (apparent) need to walk across essentially the entire tree
structure making sure that you aren't creating a loop.  This can get
rather performance piggy - even /home on my laptop has some 400K
inodes on it, and a 'find /home -type d' takes 28 seconds.  That's a *long*
time to lock and freeze a filesystem.

Where it gets *really* messy is that it isn't just mkdir that's the problem -
once you let there be more than one path from the fs root to a given directory,
it gets *really* hard to make sure that any given 'mv' command isn't going to
to screw things up (is 'mv a/b/c/d ../../w/z/b' safe? How do you know, without
examining a *lot* of stuff under a/ and ../../w/?

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