From: Mingming Cao <cmm@us.ibm.com>
Subject: Re: [PATCH 2/2] ext2: Avoid rec_len overflow with 64KB block size
Date: Thu, 18 Oct 2007 19:05:55 -0700
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Cc: Jan Kara <jack@suse.cz>, linux-ext4@vger.kernel.org,
	linux-kernel@vger.kernel.org, sho@tnes.nec.co.jp, clameter@sgi.com
To: Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <20071017210925.c3c66de5.akpm@linux-foundation.org>
Sender: linux-ext4-owner@vger.kernel.org

On Wed, 2007-10-17 at 21:09 -0700, Andrew Morton wrote:
> On Thu, 11 Oct 2007 13:18:49 +0200 Jan Kara <jack@suse.cz> wrote:
> 
> > With 64KB blocksize, a directory entry can have size 64KB which does not fit
> > into 16 bits we have for entry lenght. So we store 0xffff instead and convert
> > value when read from / written to disk.
> 
> btw, this changes ext2's on-disk format.
> 
Just to clarify this is only changes the directory entries format on
ext2/3/4 fs with 64k block size. But currently without kernel changes
ext2/3/4 does not support 64 block size.

> a) is the ext2 format documented anywhere?  If so, that document will
>    need updating.
> 

The e2fsprogs needs to be changed to sync up with this change.

Ted has a paper a while back to show ext2 disk format 
http://web.mit.edu/tytso/www/linux/ext2intro.html

The Documentation/filesystems/ext2.txt doesn't have the ext2 format
documented. That document is out-dated need to be reviewed and cleaned
up.
 
> b) what happens when an old ext2 driver tries to read and/or write this
>    directory entry?  Do we need a compat flag for it?
> 
> c) what happens when old and new ext3 or ext4 try to read/write this
>    directory entry?
> 

Without the first patch in this series: ext2 large blocksize support
patches, it fails to mount a ext2 filesystem with 64k block size. 

[PATCH 1/2] ext2:  Support large blocksize up to PAGESIZE
http://lkml.org/lkml/2007/10/1/361

So the old ext2/3/4 driver will not get access the directory entry with
64k block size format changes.


Regards,

Mingming

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