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

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Date:	Tue, 04 Oct 2011 13:15:15 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Ted Ts'o" <tytso@....edu>
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 10/4/11 1:05 PM, Ted Ts'o wrote:
> 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
> 

As long as we have a consistent, predictable, well-designed and well-understood
maximum (theoretical) size for the fs, I'm all for documenting & enforcing it.

TBH I'm still trying to get all the moving parts together in my head, between
meta_bg & bigalloc & whatnot, at these sizes.

The initialization functions are looking pretty ad-hoc to me right now. :)

-Eric
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