ext4: Fix comparison endianness problem in MMP initialization

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@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index 6b32742..7ea4ba4 100644 (file)
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -296,7 +296,8 @@ 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)