linux-ext4 - Re: [PATCH 1/2] ext2fs: add multi-mount protection (INCOMPAT

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Message-Id: <5E568C7A-6A5B-4699-9266-8A0CD2537E5F@whamcloud.com>
Date:	Fri, 29 Jul 2011 12:25:45 -0600
From:	Andreas Dilger <adilger@...mcloud.com>
To:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org,
	Johann Lombardi <johann@...mcloud.com>
Subject: Re: [PATCH 1/2] ext2fs: add multi-mount protection (INCOMPAT_MMP)

On 2011-07-29, at 11:44 AM, Bernd Schubert wrote:
> thanks a lot for sending those patches! From a previous review I still remember ext2fs_mmp_read(), see below.
> I will look through the other parts later on.
> 
>> +errcode_t ext2fs_mmp_read(ext2_filsys fs, blk64_t mmp_blk, void *buf)
>> +{
>> +	/* ext2fs_open reserves fd0,1,2 to avoid stdio collision */
>> +	if (fs->mmp_fd<= 0) {
>> +		fs->mmp_fd = open(fs->device_name, O_RDWR | O_DIRECT);
>> +		if (fs->mmp_fd<  0) {
>> +			retval = EXT2_ET_MMP_OPEN_DIRECT;
>> +			goto out;
>> +		}
>> +	}
>> +
>> +	if (ext2fs_llseek(fs->mmp_fd, mmp_blk * fs->blocksize, SEEK_SET) !=
>> +	    mmp_blk * fs->blocksize) {
>> +		retval = EXT2_ET_LLSEEK_FAILED;
>> +		goto out;
>> +	}
>> +
>> +	if (read(fs->mmp_fd, fs->mmp_cmp, fs->blocksize) != fs->blocksize) {
>> +		retval = EXT2_ET_SHORT_READ;
>> +		goto out;
>> +	}
> 
> While ext2fs_llseek() and read() works fine, I still wonder why the code does not use io_channel_read_blk64() here, similarly as io_channel_write_blk64() write in ext2fs_mmp_write().

One of the reasons for doing a direct open+seek+read here is because previous
versions of e2fsprogs did not allow O_DIRECT reads via io_channel_read_blk64(),
which is critical for the correct MMP functioning.  It cannot allow read from
a cached block, either in libext2fs or in the VM, though a cached block on the
disk is OK as long as it is visible to all hosts that are accessing it.

Cheers, Andreas
--
Andreas Dilger 
Principal Engineer
Whamcloud, Inc.



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