linux-kernel - Re: [PATCH] ext4: Fix comparison endianness problem in MMP initialization

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Date:	Sat, 8 Oct 2011 17:27:44 -0400
From:	Ted Ts'o <tytso@....edu>
To:	"Darrick J. Wong" <djwong@...ibm.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: Fix comparison endianness problem in MMP
 initialization

On Fri, Sep 30, 2011 at 12:49:00PM -0700, Darrick J. Wong wrote:
> As part of startup, the MMP initialization code does this:
> 
> mmp->mmp_seq = seq = cpu_to_le32(mmp_new_seq());
> 
> Next, mmp->mmp_seq is written out to disk, a delay happens, and then the MMP
> block is read back in and the sequence value is tested:
> 
> if (seq != le32_to_cpu(mmp->mmp_seq)) {
> 	/* fail the mount */
> 
> On a LE system such as x86, the *le32* functions do nothing and this works.
> Unfortunately, on a BE system such as ppc64, this comparison becomes:
> 
> if (cpu_to_le32(new_seq) != le32_to_cpu(cpu_to_le32(new_seq)) {
> 	/* fail the mount */
> 
> Except for a few palindromic sequence numbers, this test always causes the
> mount to fail, which makes MMP filesystems generally unmountable on ppc64.  The
> attached patch fixes this situation.
> 
> (This fix came up while testing the metadata checksumming patchset)
> 
> Signed-off-by: Darrick J. Wong <djwong@...ibm.com>

Applied, thanks.

						- Ted
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