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

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Date:	Fri, 21 Oct 2011 14:18:25 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Tao Ma <tm@....ma>, Theodore Tso <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH 5/5] ext4: Fix endian 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@...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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