linux-ext4 - Re: [PATCH 15/23] jbd2: Change disk layout for metadata checksumming

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Date:	Sat, 28 Apr 2012 10:19:33 -0400
From:	Ted Ts'o <tytso@....edu>
To:	"Darrick J. Wong" <djwong@...ibm.com>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	Martin K Petersen <martin.petersen@...cle.com>,
	Greg Freemyer <greg.freemyer@...il.com>,
	Amir Goldstein <amir73il@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Mingming Cao <cmm@...ibm.com>,
	Joel Becker <jlbec@...lplan.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-ext4@...r.kernel.org, Coly Li <colyli@...il.com>
Subject: Re: [PATCH 15/23] jbd2: Change disk layout for metadata
 checksumming

On Tue, Mar 06, 2012 at 12:49:41PM -0800, Darrick J. Wong wrote:
> @@ -177,11 +189,17 @@ typedef struct journal_block_tag_s
>  	__be32		t_blocknr;	/* The on-disk block number */
>  	__be32		t_flags;	/* See below */
>  	__be32		t_blocknr_high; /* most-significant high 32bits. */
> +	__be32		t_checksum;	/* crc32c(uuid+seq+block) */
>  } journal_block_tag_t;
>  
>  #define JBD2_TAG_SIZE32 (offsetof(journal_block_tag_t, t_blocknr_high))
>  #define JBD2_TAG_SIZE64 (sizeof(journal_block_tag_t))

There's a problem with this patch here --- we are changing the size of
journal_block_tag_t, which is an on-disk data structure.  So for
64-bit journals, this represents a format change.  This means that if
you have a 64-bit file system that needs to have its journal
recovered, if the journal was written with an older kernel, and then
we try to recover it with a new kernel, things won't be good.
Similarly, for e2fsck's recovery code, it's not going to be able to
recover 64-bit file systems using current coding, since this patch
series changes the size of JBD2_TAG_SIZE64.

What we need to do is something like this:

#define JBD2_TAG_SIZE64 (offsetof(journal_block_tag_t, t_checksum))
#define JBD2_TAG_SIZE_CSUM (sizeof(journal_block_tag_t))

And then change the code appropriately in e2fsprogs and in the kernel
to use the correct tag size depending on the journal options.

       	   	       	    	      	     - Ted
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