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Date:	Thu, 7 Apr 2011 09:40:06 -0700
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Sunil Mushran <sunil.mushran@...cle.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] ext4: Calculate and verify inode checksums

On Wed, Apr 06, 2011 at 05:52:43PM -0700, Sunil Mushran wrote:
> On 04/06/2011 03:45 PM, Darrick J. Wong wrote:
>> This patch introduces to ext4 the ability to calculate and verify inode
>> checksums.  This requires the use of a new ro compatibility flag and some
>> accompanying e2fsprogs patches to provide the relevant features in tune2fs and
>> e2fsck.
>>
>> Signed-off-by: Darrick J. Wong<djwong@...ibm.com>
>> ---
>>
>>   fs/ext4/ext4.h  |    6 ++++--
>>   fs/ext4/inode.c |   52 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>>   2 files changed, 55 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>> index 4daaf2b..8815928 100644
>> --- a/fs/ext4/ext4.h
>> +++ b/fs/ext4/ext4.h
>> @@ -617,7 +617,7 @@ struct ext4_inode {
>>   		} masix2;
>>   	} osd2;				/* OS dependent 2 */
>>   	__le16	i_extra_isize;
>> -	__le16	i_pad1;
>> +	__le16  i_checksum;		/* crc16(sb_uuid+inodenum+inode) */
>>   	__le32  i_ctime_extra;  /* extra Change time      (nsec<<  2 | epoch) */
>>   	__le32  i_mtime_extra;  /* extra Modification time(nsec<<  2 | epoch) */
>>   	__le32  i_atime_extra;  /* extra Access time      (nsec<<  2 | epoch) */
>> @@ -1338,6 +1338,7 @@ static inline void ext4_clear_state_flags(struct ext4_inode_info *ei)
>>   #define EXT4_FEATURE_RO_COMPAT_GDT_CSUM		0x0010
>>   #define EXT4_FEATURE_RO_COMPAT_DIR_NLINK	0x0020
>>   #define EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE	0x0040
>> +#define EXT4_FEATURE_RO_COMPAT_INODE_CSUM	0x0400
>>
>>   #define EXT4_FEATURE_INCOMPAT_COMPRESSION	0x0001
>>   #define EXT4_FEATURE_INCOMPAT_FILETYPE		0x0002
>> @@ -1364,7 +1365,8 @@ static inline void ext4_clear_state_flags(struct ext4_inode_info *ei)
>>   					 EXT4_FEATURE_RO_COMPAT_DIR_NLINK | \
>>   					 EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE | \
>>   					 EXT4_FEATURE_RO_COMPAT_BTREE_DIR |\
>> -					 EXT4_FEATURE_RO_COMPAT_HUGE_FILE)
>> +					 EXT4_FEATURE_RO_COMPAT_HUGE_FILE |\
>> +					 EXT4_FEATURE_RO_COMPAT_INODE_CSUM)
>>
>>   /*
>>    * Default values for user and/or group using reserved blocks
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index 1a86282..dc76f19 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -42,6 +42,7 @@
>>   #include<linux/printk.h>
>>   #include<linux/slab.h>
>>   #include<linux/ratelimit.h>
>> +#include<linux/crc16.h>
>>
>>   #include "ext4_jbd2.h"
>>   #include "xattr.h"
>> @@ -52,6 +53,40 @@
>>
>>   #define MPAGE_DA_EXTENT_TAIL 0x01
>>
>> +static __le16 ext4_inode_csum(struct inode *inode, struct ext4_inode *raw)
>> +{
>> +	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
>> +	struct ext4_inode_info *ei = EXT4_I(inode);
>> +	int offset = offsetof(struct ext4_inode, i_checksum);
>> +	__le32 inum = cpu_to_le32(inode->i_ino);
>> +	__u16 crc = 0;
>> +
>> +	if (EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb,
>> +		EXT4_FEATURE_RO_COMPAT_INODE_CSUM)&&
>> +	    le16_to_cpu(raw->i_extra_isize)>= 4) {
>> +		crc = crc16(~0, sbi->s_es->s_uuid, sizeof(sbi->s_es->s_uuid));
>> +		crc = crc16(crc, (__u8 *)&inum, sizeof(inum));
>> +		crc = crc16(crc, (__u8 *)raw, offset);
>> +		offset += sizeof(raw->i_checksum); /* skip checksum */
>> +		/* for checksum of struct ext4_inode do the rest...*/
>> +		if (ei->i_extra_isize>  4)
>> +			crc = crc16(crc, (__u8 *)raw + offset,
>> +				    ei->i_extra_isize - 4);
>> +	}
>> +
>> +	return cpu_to_le16(crc);
>> +}
>> +
>> +static int ext4_inode_csum_verify(struct inode *inode, struct ext4_inode *raw)
>> +{
>> +	if (EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb,
>> +		EXT4_FEATURE_RO_COMPAT_INODE_CSUM)&&
>> +	    (raw->i_checksum != ext4_inode_csum(inode, raw)))
>> +		return 0;
>> +
>> +	return 1;
>> +}
>> +
>>   static inline int ext4_begin_ordered_truncate(struct inode *inode,
>>   					      loff_t new_size)
>>   {
>> @@ -4802,7 +4837,8 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
>>   	struct ext4_inode *raw_inode;
>>   	struct ext4_inode_info *ei;
>>   	struct inode *inode;
>> -	journal_t *journal = EXT4_SB(sb)->s_journal;
>> +	struct ext4_sb_info *sbi = EXT4_SB(sb);
>> +	journal_t *journal = sbi->s_journal;
>>   	long ret;
>>   	int block;
>>
>> @@ -4916,6 +4952,14 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
>>   	} else
>>   		ei->i_extra_isize = 0;
>>
>> +	if (!ext4_inode_csum_verify(inode, raw_inode)) {
>> +		EXT4_ERROR_INODE(inode, "checksum invalid (%u != %u)",
>> +		       le16_to_cpu(ext4_inode_csum(inode, raw_inode)),
>> +		       le16_to_cpu(raw_inode->i_checksum));
>> +		ret = -EIO;
>> +		goto bad_inode;
>> +	}
>> +
>>   	EXT4_INODE_GET_XTIME(i_ctime, inode, raw_inode);
>>   	EXT4_INODE_GET_XTIME(i_mtime, inode, raw_inode);
>>   	EXT4_INODE_GET_XTIME(i_atime, inode, raw_inode);
>> @@ -5138,6 +5182,12 @@ static int ext4_do_update_inode(handle_t *handle,
>>   			raw_inode->i_version_hi =
>>   			cpu_to_le32(inode->i_version>>  32);
>>   		raw_inode->i_extra_isize = cpu_to_le16(ei->i_extra_isize);
>> +
>> +		if (EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb,
>> +			EXT4_FEATURE_RO_COMPAT_INODE_CSUM)&&
>> +		    EXT4_FITS_IN_INODE(raw_inode, ei, i_checksum))
>> +			raw_inode->i_checksum =
>> +				ext4_inode_csum(inode, raw_inode);
>>   	}
>>
>>   	BUFFER_TRACE(bh, "call ext4_handle_dirty_metadata");
>
> You may want to look into jbd2 buffer triggers. struct jbd2_buffer_trigger_type
> Instead of computing checksum on every update, it allows one to compute it
> just before the journal write. More efficient.

Yes, I see that jbd2 has triggers, looks like a nifty feature.  I suppose if I
went with that approach I'd still have to calculate the checksum in
ext4_do_update_inode in the nojournal case, and in the journal case I'd write a
trigger that would figure out which inodes in a given buffer are actually dirty
and compute their checksums.

That said, I haven't really quantified the performance impact of this naive
approach yet, so I wonder -- did you see a similar scenario with ocfs2, and
what kind of performance increase did you get by adapting the code to use the
jbd2 trigger?  If there's potentially a large increase, it would be interesting
to apply the same conversion to the group descriptor checksumming code too.

--D
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