Re: [PATCH 02/27] allow hard links to directories, opt-in for any filesystem

Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
Joshua Hudson wrote:
Patch seems to work, might want more testing.
It probably should not be applied without a discussion, especially
as no filesystem in kernel tree wants this. I am working on a fs that does.

This is backwards I think. This is not disallowed because there are
no filesystems that want it. Linux doesn't want it so it is disallowed
by the vfs.

Right.

You have to put forward a case for why we want it, rather than show us
your filesystem that "wants" it. Right?

Nope. The "why a FS might want it" part is pretty clear (we do have
symlinks to directories as a poor man's substitute, after all), the "why it
can't be allowed" part is the tricky one...

- It creates the possibility of loops ==> The garbage collection of unused
stuff can't be done just by reference counts (as today), and gets very
hairy... and needs a /lot/ of memory.
- Loop detection/breaking in a general graph /can't/ be done while it is
being updated, and they thake a long time (and lots of memory)
- Can't just assume that by locking in the directory/subdirectory/... order
no deadlocks are possible, and traversing the filesystem for surgery gets
to be one operation at a time, not concurrent as today
- A while back (in one of the recurrent ReiserFS flamewars Re: Files as
directories, which creates exactly the same situation when linking to a
file-is-a-directory) somebody (Linus? Ted T'so?) showed that certain
simple operations would potentially require exponential time or memory (I
forget which), and thus the idea was out of the question.

Sure, the situations that give rise to the problems are rather clear-cut,
and could be disallowed, but the result for the user would have seemingly
random restrictions. IMVHO it is much better to have a restricted system
with a simple conceptual model than a more relaxed one, but with hard to
understand corner cases it doesn't allow.
--
Dr. Horst H. von Brand User #22616 counter.li.org
Departamento de Informatica Fono: +56 32 654431
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Casilla 110-V, Valparaiso, Chile Fax: +56 32 797513
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