Introduce SEEK_DATA/SEEK_HOLE to XFS V5

[Ben Myers]

Hey Jeff,

On Wed, Jan 11, 2012 at 09:43:18AM -0600, Ben Myers wrote:
> Here are a few additional minor comments from yesterday.
> 
> I'm looking forward to seeing your next version, and I'm still working
> through this one.
> 
> I would like to suggest that you split this into two patches.  The first
> patch should be the 'simple' implementation that that you began with
> that only looks at extents, and assumes that unwritten extents contain
> data.  The second patch can remove the assumption that unwritten extents
> contain data, and go over pages over the extent to determine if it is
> clean.  I feel we have a better chance of coming to consensus that the
> first patch is correct in the near term, and then we can move on to the
> more complicated matter of whether the unwritten extent can be treated
> as a hole safe in the knowledge that the initial implementation was
> awesome.

Ok, since I'm the jackass who is asking you to do the extra work I'll
try to be of assistance.  Understand that at this point I'm trying to
make sure that I understand your code fully.  I'm not trying to give you
a hard time or make your life miserable.

Here I am assuming that we'll treat unwritten extents as containing data
and leaving the enhancement of probing unwritten extents for later.

This is a table of some of the results from xfs_bmapi_read, and what
should be done in each situation.

SEEK_DATA:

Where nmap = 0:
return ENXIO. 	* maybe not possible, unless len = 0?

Where nmap = 1:
map[0]
written				data @ offset
delay				data @ offset
unwritten			data @ offset
hole				return ENXIO?	* empty file?

Where nmap = 2:
map[0]		map[1]
written		written		data @ offset
written		delay		data @ offset
written		unwritten	data @ offset
written		hole		data @ offset
delay		written		data @ offset
delay		delay		data @ offset	* maybe not possible?
delay		unwritten	data @ offset
delay		hole		data @ offset
unwritten	written		data @ offset
unwritten	delay		data @ offset
unwritten	unwritten	data @ offset
unwritten	hole		data @ offset
hole		written		data @ map[1].br_startoff
hole		delay		data @ map[1].br_startoff
hole		unwritten	data @ map[1].br_startoff
hole		hole		* not possible

(DELAYSTARTBLOCK and HOLESTARTBLOCK are both 'isnullstartblock')

written:
(!isnullstartblock(map.br_startblock) && map.br_state == XFS_EXT_NORMAL)	
delay:
map.br_startblock == DELAYSTARTBLOCK

unwritten:
map.br_state == XFS_EXT_UNWRITTEN

hole:
map.br_startblock == HOLESTARTBLOCK

xfs_seek_data(file, startoff)
{
	loff_t	offset;
	int	error;

	take ilock

	isize = i_size_read

	start_fsb = XFS_B_TO_FSBT(startoff)
	end_fsb = XFS_B_TO_FSB(i_size)	# inode size

	error = xfs_bmapi_read(map, &nmap)
	if (error) 
		goto out_unlock;

	if (nmap == 0) {
		/*
		 * return an error.  I'm not sure that this necessarily
		 * means we're reading after EOF, since it looks like
		 * xfs_bmapi_read would return one hole in that case.
		 */

		error = ERROR /* EIO? */
		goto out_unlock
	}

	/* check map[0] first */
	if (map[0].br_state == XFS_EXT_NORMAL &&
	    !isnullstartblock(map[0].br_startblock) {
		/*
		 * startoff is already within data.  remember
		 * that it can anywhere within start_fsb
		 */
		offset = startoff
	} else if (map[0].br_startblock == DELAYSTARTBLOCK) {
		offset = startoff
	} else if (map[0].br_state == XFS_EXT_UNWRITTEN) {
		offset = startoff;
	} else if (map[0].br_startblock == HOLESTARTBLOCK) {
		if (nmap == 1) {
			/*
			 * finding a hole in map[0] and nothing in
			 * map[1] probably means that we are reading
			 * after eof
			 */
			ASSERT(startoff >= isize)
			error = ENXIO
			goto out_unlock
		}

		/*
		 * we have two mappings, and need to check map[1] to see
		 * if there is data.
		 */
		if (map[1].br_state == XFS_EXT_NORMAL &&
		    !isnullstartblock(map[1].br_startblock)) {
			offset = XFS_FSB_TO_B(map[1].br_startoff);
		} else if (map[1].br_startblock == DELAYSTARTBLOCK) {
			offset = XFS_FSB_TO_B(map[1].br_startoff);
		} else if (map[1].br_state == XFS_EXT_UNWRITTEN) {
			offset = XFS_FSB_TO_B(map[1].br_startoff);
		} else if (map[1].br_startblock == HOLESTARTBLOCK) {
			/*
			 * this should never happen, but we could
			 */
			ASSERT(startoff >= isize);
			error = ENXIO
			/* BUG(); */
		} else {
			offset = startoff
			/* BUG(); */
		}
	} else {
		offset = startoff
		/* BUG(); */
	}
out_unlock:
	drop ilock
	if (error)
		return -error;

	return offset;
}

I think that is sufficiently straightforward that even I can understand
it, or am I off my rocker?  IMO it's not that bad that we have to write
the if/else to determine extent type twice and that there is some
duplication when setting the offset.  When you come back to enhance it
further by probing unwritten extents I think a goto would probably be
more readable than trying to shoehorn this into a for/do, but that's
just me.

Jeff, I hope that doesn't ruffle any feathers.  I know I came to the
party a bit late.  After a break I am going to go look at your code for
xfs_seek_data again.  I think I'll understand it better now.  After that
I am going to look into SEEK_HOLE... 

Regards,
Ben