linux-ext4 - Re: [PATCH 11/37] libext2fs: Create the inode bitmap checksum

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Message-Id: <51D918B5-3B98-43F1-B384-D4AE3B77E6C3@dilger.ca>
Date:	Wed, 14 Sep 2011 14:00:18 -0600
From:	Andreas Dilger <adilger.kernel@...ger.ca>
To:	djwong@...ibm.com
Cc:	"Ted Ts'o" <tytso@....edu>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Amir Goldstein <amir73il@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>, linux-ext4@...r.kernel.org,
	Coly Li <colyli@...il.com>
Subject: Re: [PATCH 11/37] libext2fs: Create the inode bitmap checksum

On 2011-09-14, at 1:31 PM, Darrick J. Wong wrote:
> On Wed, Sep 14, 2011 at 01:02:59PM -0400, Ted Ts'o wrote:
>> On Wed, Aug 31, 2011 at 05:36:22PM -0700, Darrick J. Wong wrote:
>>> diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h
>>> index 1f08673..367bfdf 100644
>>> --- a/lib/ext2fs/ext2_fs.h
>>> +++ b/lib/ext2fs/ext2_fs.h
>>> @@ -169,7 +169,8 @@ struct ext4_group_desc
>>> 	__u16	bg_free_inodes_count_hi;/* Free inodes count MSB */
>>> 	__u16	bg_used_dirs_count_hi;	/* Directories count MSB */
>>> 	__u16	bg_itable_unused_hi;	/* Unused inodes count MSB */
>>> -	__u32	bg_reserved2[3];
>>> +	__u32	bg_inode_bitmap_csum;	/* crc32c(uuid+group+ibitmap) */
>>> +	__u32	bg_reserved2[2];
>>> };
>> 
>> 
>> One of the reasons why I like to coalesce the patches to the data
>> structures into their own separate commit, is it's hard when I'm
>> reviewing individual patches in a mail reader what's going on from a
>> big picture spective.  (Heck, even just *finding* the patches that
>> modify the on-disk format is hard....)
>> 
>> But as near as I can tell, your patch series only uses one of the
>> 32-bit fields in bg_reserved.  Is there a good reason why
>> bg_inode_bitmap_csum can't also used one of the two fields in
>> bg_reserved?  That way we get two 32-bit checksums for both struct
>> ext2_group_desc and struct ext4_group_desc.  Is there a third 32-bit
>> per-block group checksum I'm forgetting about? 
> 
> No, but I was under the impression that Amir was using one of bg_reserved for
> snapshots.  If that's not true, I'll move both bitmap checksums to bg_reserved.

He was, AND the first bg_reserved2 field as well, which the above use
would conflict with.  See my other patch.

Cheers, Andreas





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