linux-ext4 - Re: [PATCH 2/3] mke2fs: reduce the range of cluster-size

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Date:	Mon, 14 Jan 2013 14:07:43 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Zheng Liu <gnehzuil.liu@...il.com>, linux-ext4@...r.kernel.org,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [PATCH 2/3] mke2fs: reduce the range of cluster-size

On 2013-01-14, at 2:03 PM, Theodore Ts'o wrote:
> On Sun, Jan 13, 2013 at 05:08:14PM +0800, Zheng Liu wrote:
>> From: Zheng Liu <wenqing.lz@...bao.com>
>> 
>> There are two bugs need to be fixed, which are about cluster-size.
>> Now the range of cluster-size is from 1024 to 512M bytes.  Although
>> with '-C 1024', the cluster-size will be 4096 after making a
>> filesystem because in ext2fs_initialize() set_field() needs to check
>> 'param->s_log_cluster_size' and s_log_cluster_size is 0 as
>> cluster-size is 1024.  Then s_log_cluster_size will be assigned to
>> s_log_block_size+4.  So we never set cluster-size to 1024.
>> 
>> Another bug is that when cluster-size is 512M EXT2FS_C2B will return
>> 0.  So s_blocks_per_group will be assigned to zero and we will meet
>> a 'division by zero' error.
> 
> There are a couple of things going on here.  The first is that it
> makes no senes when the cluster size is less than or equal to the
> block size.  (Actually, nothing bad should happen in the case when
> cluster size == block size, but if the user specified the bigalloc
> feature, that's something which they almost certainly don't want.)
> 
> So the more general check is we should be complaining if the cluster
> size is <= the block size.  That is, the combination of -b 4096 and
> -C 2048 makes no sense, either.
> 
> Also, there's technically nothing wrong with a cluster size of 512MB.
> The problem is in how we calculate the default number of clusters per
> group --- if it translates to a number of blocks per group which
> overals 2**32, that's when we run into problems.
> 
> Which leads to another bug in the current mke2fs command.  The range
> checking for the -g (which allows you to specify the number of blocks
> per group is bogus in the case when the bigalloc feature is enabled).
> I think the best way of fixing this is to document that the -g option
> specifies the number of clusters per block if the bigalloc feature is
> enabled.

Presumably you mean "the -g option specifies the number of clusters
_per_group_ if the bigalloc feature is enabled"?

Cheers, Andreas





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