Re: [PATCH 05/24] set_cur: Add support to read from external log device

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On Tue, May 23, 2023 at 09:48:07 AM -0700, Darrick J. Wong wrote:
> On Tue, May 23, 2023 at 02:30:31PM +0530, Chandan Babu R wrote:
>> This commit changes set_cur() to be able to read from external log
>> devices. This is required by a future commit which will add the ability to
>> dump metadata from external log devices.
>> 
>> Signed-off-by: Chandan Babu R <chandan.babu@xxxxxxxxxx>
>> ---
>>  db/io.c   | 22 +++++++++++++++-------
>>  db/type.c |  2 ++
>>  db/type.h |  2 +-
>>  3 files changed, 18 insertions(+), 8 deletions(-)
>> 
>> diff --git a/db/io.c b/db/io.c
>> index 3d2572364..e8c8f57e2 100644
>> --- a/db/io.c
>> +++ b/db/io.c
>> @@ -516,12 +516,13 @@ set_cur(
>>  	int		ring_flag,
>>  	bbmap_t		*bbmap)
>>  {
>> -	struct xfs_buf	*bp;
>> -	xfs_ino_t	dirino;
>> -	xfs_ino_t	ino;
>> -	uint16_t	mode;
>> +	struct xfs_buftarg	*btargp;
>> +	struct xfs_buf		*bp;
>> +	xfs_ino_t		dirino;
>> +	xfs_ino_t		ino;
>> +	uint16_t		mode;
>>  	const struct xfs_buf_ops *ops = type ? type->bops : NULL;
>> -	int		error;
>> +	int			error;
>>  
>>  	if (iocur_sp < 0) {
>>  		dbprintf(_("set_cur no stack element to set\n"));
>> @@ -534,7 +535,14 @@ set_cur(
>>  	pop_cur();
>>  	push_cur();
>>  
>> +	btargp = mp->m_ddev_targp;
>> +	if (type->typnm == TYP_ELOG) {
>
> This feels like a layering violation, see below...
>
>> +		ASSERT(mp->m_ddev_targp != mp->m_logdev_targp);
>> +		btargp = mp->m_logdev_targp;
>> +	}
>> +
>>  	if (bbmap) {
>> +		ASSERT(btargp == mp->m_ddev_targp);
>>  #ifdef DEBUG_BBMAP
>>  		int i;
>>  		printf(_("xfs_db got a bbmap for %lld\n"), (long long)blknum);
>> @@ -548,11 +556,11 @@ set_cur(
>>  		if (!iocur_top->bbmap)
>>  			return;
>>  		memcpy(iocur_top->bbmap, bbmap, sizeof(struct bbmap));
>> -		error = -libxfs_buf_read_map(mp->m_ddev_targp, bbmap->b,
>> +		error = -libxfs_buf_read_map(btargp, bbmap->b,
>>  				bbmap->nmaps, LIBXFS_READBUF_SALVAGE, &bp,
>>  				ops);
>>  	} else {
>> -		error = -libxfs_buf_read(mp->m_ddev_targp, blknum, len,
>> +		error = -libxfs_buf_read(btargp, blknum, len,
>>  				LIBXFS_READBUF_SALVAGE, &bp, ops);
>>  		iocur_top->bbmap = NULL;
>>  	}
>> diff --git a/db/type.c b/db/type.c
>> index efe704456..cc406ae4c 100644
>> --- a/db/type.c
>> +++ b/db/type.c
>> @@ -100,6 +100,7 @@ static const typ_t	__typtab_crc[] = {
>>  	{ TYP_INODE, "inode", handle_struct, inode_crc_hfld,
>>  		&xfs_inode_buf_ops, TYP_F_CRC_FUNC, xfs_inode_set_crc },
>>  	{ TYP_LOG, "log", NULL, NULL, NULL, TYP_F_NO_CRC_OFF },
>> +	{ TYP_ELOG, "elog", NULL, NULL, NULL, TYP_F_NO_CRC_OFF },
>
> It strikes me as a little odd to create a new /metadata type/ to
> reference the external log.  If we someday want to add a bunch of new
> types to xfs_db to allow us to decode/fuzz the log contents, wouldn't we
> have to add them twice -- once for decoding an internal log, and again
> to decode the external log?  And the only difference between the two
> would be the buftarg, right?  The set_cur caller needs to know the
> daddr already, so I don't think it's unreasonable for the caller to have
> to know which buftarg too.
>
> IOWs, I think set_cur ought to take the buftarg, the typ_t, and a daddr
> as explicit arguments.  But maybe others have opinions?

You are right about the requirement to add two entries for each possible
operation related to the log. 
>
> e.g. rename set_cur to __set_cur and make it take a buftarg, and then:
>
> int
> set_log_cur(
> 	const typ_t	*type,
> 	xfs_daddr_t	blknum,
> 	int		len,
> 	int		ring_flag,
> 	bbmap_t		*bbmap)
> {
> 	if (!mp->m_logdev_targp->bt_bdev ||
> 	    mp->m_logdev_targp->bt_bdev == mp->m_ddev_targp->bt_bdev) {
> 		printf(_("external log device not loaded, use -l.\n"));
> 		return ENODEV;
> 	}
>
> 	__set_cur(mp->m_logdev_targp, type, blknum, len, ring_flag, bbmap);
> 	return 0;
> }
>
> and then metadump can do something like ....
>
> 	error = set_log_cur(&typtab[TYP_LOG], 0,
> 			mp->m_sb.sb_logblocks * blkbb, DB_RING_IGN, NULL);

The above suggestion looks correct to me. I will include this change unless
others have any objections to it.

-- 
chandan



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