On 2011-09-30, at 1:49 PM, 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)
Thanks also for the catch, you are completely right. We've never run
this code on a big-endian system in all the years we had it.
> Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> ---
>
> 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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Cheers, Andreas
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