From: Andreas Dilger Subject: Re: [PATCH] ext4: Fix comparison endianness problem in MMP initialization Date: Fri, 30 Sep 2011 15:12:02 -0600 Message-ID: <37339E8E-181A-4245-86BD-64EE47A54926@dilger.ca> References: <20110930194900.GY12086@tux1.beaverton.ibm.com> Mime-Version: 1.0 (Apple Message framework v1084) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: "Theodore Ts'o" , linux-kernel , linux-ext4 To: djwong@us.ibm.com Return-path: Received: from idcmail-mo1so.shaw.ca ([24.71.223.10]:53415 "EHLO idcmail-mo1so.shaw.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754981Ab1I3VME (ORCPT ); Fri, 30 Sep 2011 17:12:04 -0400 In-Reply-To: <20110930194900.GY12086@tux1.beaverton.ibm.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 > --- > > 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) > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html Cheers, Andreas