linux-ext4 - Re: [PATCH 2/2] libext2fs: fix bad cast which causes problems for file systems

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Date:	Tue, 4 Oct 2011 14:05:55 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Eric Sandeen <sandeen@...deen.net>
Cc:	"Richard W.M. Jones" <rjones@...hat.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 2/2] libext2fs: fix bad cast which causes problems for
 file systems > 512EB

On Tue, Oct 04, 2011 at 06:47:12AM -0500, Eric Sandeen wrote:
> On 10/3/11 11:27 PM, Theodore Ts'o wrote:
> > If the number of block groups exceeds 2**32, a bad cast would lead to
> > a bogus "Not enough space to build proposed filesystem while setting
> > up superblock" failure.
> 
> It's the proper cast now, but I don't think it fixes the problem, since they
> are both __u32...

Hmm, yes.

And to be quite honest I'm not sure it's worth fixing.  2**32 block
groups gets us up to 2**59 bytes assuming 4k blocks.  The theoretical
maximum given the current extent tree format is 2**60 assuming 4k
blocks.  So changing dgrp_t to be 64-bits just to get that last power
of two (i.e., from 512EB to a full PB) doesn't seem worth it.  Simply
using a bigalloc cluster size of 8k would make the problem go away
(and arguably we'd probably want a large cluster size if someone
wanted to create a file system that big anyway).

So maybe we should just check to see if the required number of block
groups is greater than 2**32, and if so, give an error.

       	  	       	      	  - Ted
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