linux-ext4 - Re: [PATCH 5/5] ext4: Fix endian problem in MMP initialization

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Date:	Sat, 22 Oct 2011 11:25:31 -0600
From:	Andreas Dilger <aedilger@...il.com>
To:	"Darrick J. Wong" <djwong@...ibm.com>
Cc:	Tao Ma <tm@....ma>, Theodore Tso <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] ext4: Fix endian problem in MMP initialization

You can add my

 Acked-by: Andreas Dilger <adilger@...ger.ca>

to this. 

Cheers, Andreas

On 2011-10-21, at 3:18 PM, "Darrick J. Wong" <djwong@...ibm.com> 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.
> 
> Signed-off-by: Darrick J. Wong <djwong@...ibm.com>
> ---
> fs/ext4/mmp.c |    3 ++-
> 1 files changed, 2 insertions(+), 1 deletions(-)
> 
> 
> diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
> index 9bdef3f..a7a4986 100644
> --- a/fs/ext4/mmp.c
> +++ b/fs/ext4/mmp.c
> @@ -295,7 +295,8 @@ skip:
>    /*
>     * write a new random sequence number.
>     */
> -    mmp->mmp_seq = seq = cpu_to_le32(mmp_new_seq());
> +    seq = mmp_new_seq();
> +    mmp->mmp_seq = cpu_to_le32(seq);
> 
>    retval = write_mmp_block(bh);
>    if (retval)
> 
> --
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