Re: reiser4 plugins

[Jan Harkes <jaharkes@cs.cmu.edu>]

On Wed, Jul 06, 2005 at 09:33:13PM -0500, David Masover wrote:
> And speaking of which, the only doomsday scenario (running out of RAM) 
> that I can think of with this scheme is if we have a ton of hardlinks to 
> the same file and we try to move one of them.  But this scales linearly 
> with the number of hardlinks, I think.  Maybe not quite, but certainly 
> not exponentially.
> 
> The only other doomsday scenario is if we have a ludicrously deep tree.

rename(a/b, c/b), if a and c are identical.

End result, either you deadlock trying to lock the same object twice, or
you end up removing b since the target of a rename is unlinked.

The VFS uses dentries, there is one per hardlinked object, and they have
a single parent only. So a/b and c/b are represented by the same inode,
but they have completely different dentry-aliases associated with them. 

Similar things for removal, we can safely remove a file, but not a
directory that still has children, if you have 'meta-files' hanging of a
file, you'd have to get rid of the metadata objects first.

If you want to use a file as a directory, it probably will need the same
restrictions as a directory if you expect the dcache and VFS locking to
work correctly. So that means, _no hardlinks to files_ (the file system
could internally implement copy-on-write type links, or use a content
addressable storage to deal with diskspace issues) and the file system
probably has to d_unhash/destroy metadata objects before it can unlink
the file object, etc.

> To make this work in real usage, not DOS testing, we really need both of 
> those, and even then I'm not sure it can work.  What's the maximum 
> number of hardlinks supported to a single file?

I believe nlink is a 16-bit value, so that would be either 32K or 64K
depending on signedness.

> What's the maximum tree depth?

Last time I checked, PATH_MAX is 4096, so that would be about 2048
single character directory names.

> Can these be limited to prevent actual DOS attempts?

Is that the 'nobody needs more than 64KB of memory' kind of DOS?

Jan

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