Message-ID: <20140210233258.GX13647@dastard>
Date:	Tue, 11 Feb 2014 10:32:58 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Namjae Jeon <linkinjeon@...il.com>
Cc:	viro@...iv.linux.org.uk, bpm@....com, tytso@....edu,
	adilger.kernel@...ger.ca, jack@...e.cz, mtk.manpages@...il.com,
	linux-fsdevel@...r.kernel.org, xfs@....sgi.com,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	Namjae Jeon <namjae.jeon@...sung.com>,
	Ashish Sangwan <a.sangwan@...sung.com>
Subject: Re: [PATCH RESEND 2/10] xfs: Add support FALLOC_FL_COLLAPSE_RANGE
 for fallocate

On Sun, Feb 02, 2014 at 02:44:11PM +0900, Namjae Jeon wrote:
> From: Namjae Jeon <namjae.jeon@...sung.com>
> 
> Add support FALLOC_FL_COLLAPSE_RANGE for fallocate.
> 
> Signed-off-by: Namjae Jeon <namjae.jeon@...sung.com>
> Signed-off-by: Ashish Sangwan <a.sangwan@...sung.com>

A more detailed description would be nice for the change logs.
.....

> +	while (nexts++ < num_exts &&
> +	       *current_ext <  XFS_IFORK_NEXTENTS(ip, whichfork)) {
> +
> +		gotp = xfs_iext_get_ext(ifp, *current_ext);
> +		xfs_bmbt_get_all(gotp, &got);
> +		startoff = got.br_startoff - offset_shift_fsb;
> +
> +		/*
> +		 * Before shifting extent into hole, make sure that the hole
> +		 * is large enough to accomodate the shift.
> +		 */
> +		if (*current_ext) {
> +			xfs_bmbt_get_all(xfs_iext_get_ext(ifp,
> +						*current_ext - 1), &left);
> +
> +			if (startoff < left.br_startoff + left.br_blockcount)
> +				error = XFS_ERROR(EINVAL);
> +
> +		} else if (startoff > xfs_bmbt_get_startoff(gotp)) {
> +			/* Hole is at the start but not large enough */
> +			error = XFS_ERROR(EINVAL);
> +		}

This second branch seems wrong to me:

	startoff = got.br_startoff - offset_shift_fsb;
and
	got.br_startoff = xfs_bmbt_get_startoff(gotp)).

I'm not 100% sure what you are trying to check in this case -
perhaps some basic ascii art to describe the two cases is in order
here:

	left	hole		got
	+-------+hhhhhhhhhhhhhhh+---------+
	LS      LE              GS        GE
		HS              HE

The first is checking that GS - offset_shift_fsb is greater than LE.
i.e the shift doesn't overrun the hole betwenn LE and GS.

	left	hole		got
	+-------+hhhhhhhhhhhhhhh+---------+
	LS      LE              GS        GE
		HS              HE
	+-------+hhhhhhh+---------+
	LS      LE      GS'       GE'
		HS      HE'

The second I can't visualise from the code or comment....


> +
> +		if (error)
> +			goto del_cursor;
> +
> +		if (cur) {
> +			error = xfs_bmbt_lookup_eq(cur,
> +					got.br_startoff,
> +					got.br_startblock,
> +					got.br_blockcount,
> +					&i);

Whitespace comment - a more compact form is the typical XFS
convention if it will fit in 80 columns:

			error = xfs_bmbt_lookup_eq(cur, got.br_startoff,
						   got.br_startblock,
						   got.br_blockcount, &i);

> +			if (error)
> +				goto del_cursor;
> +			XFS_WANT_CORRUPTED_GOTO(i == 1, del_cursor);
> +		}
> +
> +		/* Check if we can merge 2 adjacent extents */
> +		if (*current_ext &&
> +		    left.br_startoff + left.br_blockcount == startoff &&
> +		    left.br_startblock + left.br_blockcount ==
> +				got.br_startblock &&
> +		    left.br_state == got.br_state &&
> +		    left.br_blockcount + got.br_blockcount <= MAXEXTLEN) {
> +			blockcount = left.br_blockcount +
> +				xfs_bmbt_get_blockcount(gotp);

				got.br_blockcount?

> +			xfs_iext_remove(ip, *current_ext, 1, 0);
> +			if (cur) {
> +				error = xfs_btree_delete(cur, &i);
> +				if (error)
> +					goto del_cursor;
> +				XFS_WANT_CORRUPTED_GOTO(i == 1, del_cursor);
> +			}
> +			XFS_IFORK_NEXT_SET(ip, whichfork,
> +				XFS_IFORK_NEXTENTS(ip, whichfork) - 1);
> +			gotp = xfs_iext_get_ext(ifp, --*current_ext);
> +			xfs_bmbt_get_all(gotp, &got);
> +
> +			/* Make cursor point to the extent we will update */
> +			if (cur) {
> +				error = xfs_bmbt_lookup_eq(cur,
> +				got.br_startoff,
> +				got.br_startblock,
> +				got.br_blockcount,
> +				&i);

whitespace.

> +				if (error)
> +					goto del_cursor;
> +				XFS_WANT_CORRUPTED_GOTO(i == 1, del_cursor);
> +			}
> +
> +			xfs_bmbt_set_blockcount(gotp, blockcount);
> +			got.br_blockcount = blockcount;
> +			goto bmbt_update;
> +		}
> +
> +		/* We have to update the startoff */
> +		xfs_bmbt_set_startoff(gotp, startoff);
> +		got.br_startoff = startoff;
> +
> +bmbt_update:

Use an } else {} for this, and the goto can be removed.

....
>  /*
> + * xfs_collapse_file_space()
> + *      This routine frees disk space and shift extent for the given file.
> + *
> + * RETURNS:
> + *      0 on success
> + *      errno on error
> + *
> + */
> +int
> +xfs_collapse_file_space(
> +	struct xfs_inode	*ip,
> +	xfs_off_t		offset,
> +	xfs_off_t		len)
> +{
> +	int			done = 0;
> +	struct xfs_mount	*mp = ip->i_mount;
> +	struct xfs_trans	*tp;
> +	int			error;
> +	xfs_extnum_t		current_ext = 0;
> +	struct xfs_bmap_free	free_list;
> +	xfs_fsblock_t		first_block;
> +	int			committed;
> +	xfs_fileoff_t		start_fsb;
> +	xfs_fileoff_t		shift_fsb;
> +
> +	ASSERT(xfs_isilocked(ip, XFS_IOLOCK_EXCL));
> +
> +	trace_xfs_collapse_file_space(ip);
> +
> +	start_fsb = XFS_B_TO_FSB(mp, offset + len);
> +	shift_fsb = XFS_B_TO_FSB(mp, len);
> +
> +	/*
> +	 * The first thing we do is to free data blocks in the specified range
> +	 * by calling xfs_free_file_space(). It would also sync dirty data
> +	 * and invalidate page cache over the region on which collapse range
> +	 * is working.
> +	 */

This can probably go in the function header as part of describing
the overall algorithm the code is using.

> +	error = xfs_free_file_space(ip, offset, len);
> +	if (error)
> +		return error;
> +
> +	while (!error && !done) {
> +		tp = xfs_trans_alloc(mp, XFS_TRANS_DIOSTRAT);
> +		tp->t_flags |= XFS_TRANS_RESERVE;
> +		/*
> +		 * We would need to reserve permanent block for transaction.
> +		 * This will come into picture when after shifting extent into
> +		 * hole we found that adjacent extents can be merged which
> +		 * may lead to freeing of a block during record update.
> +		 */
> +		error = xfs_trans_reserve(tp, &M_RES(mp)->tr_write,
> +				XFS_DIOSTRAT_SPACE_RES(mp, 0), 0);
> +		if (error) {
> +			ASSERT(error == ENOSPC || XFS_FORCED_SHUTDOWN(mp));
> +			xfs_trans_cancel(tp, 0);
> +			break;
> +		}
> +
> +		xfs_ilock(ip, XFS_ILOCK_EXCL);
> +		error = xfs_trans_reserve_quota(tp, mp, ip->i_udquot,
> +				ip->i_gdquot, ip->i_pdquot,
> +				XFS_DIOSTRAT_SPACE_RES(mp, 0), 0,
> +				XFS_QMOPT_RES_REGBLKS);
> +		if (error)
> +			goto out;
> +
> +		xfs_trans_ijoin(tp, ip, 0);
> +
> +		xfs_bmap_init(&free_list, &first_block);
> +
> +		/*
> +		 * We are using the write transaction in which max 2 bmbt
> +		 * updates are allowed
> +		 */

Right, but you've only reserved blocks for a single BMBT split
through XFS_DIOSTRAT_SPACE_RES(). In cases of allocation, adjacent
offset allocation is guaranteed to only require one split at most
and hence it's safe from that perspective. However, I can see how a
shift can require a split on the first extent move, and a merge on
the second. e.g:


		left		middle		right
before		maxrecs		minrecs + 1	minrecs
first shift	maxrecs + 1	minrecs		minrecs
		split
		maxrecs / 2	minrecs		minrecs
second shift
		maxrecs/2 + 1	minrecs - 1	minrecs
				merge		merge
				minrecs*2 - 1	(freed)

So the question is whether the transaction reservations (both log
space and block allocation requirements) are covered.

> +		error = xfs_bmap_shift_extents(tp, ip, &done, start_fsb,
> +				shift_fsb, &current_ext,
> +				&first_block, &free_list, 2);

And that should really have a #define associated with it. ie.:

#define XFS_BMAP_MAX_SHIFT_EXTENTS	2

And document the constraints that lead to that number with the
definition.

Overall, all I'm really looking for here is sufficient comments to
document the constraints the code is operating under. Functionally
the code looks correct (apart from the branch above I can't work
out). Mostly I just want to make sure that in a couple of
years time I don't have to work it all out from first principles
again. ;)

Cheers,

Dave.

-- 
Dave Chinner
david@...morbit.com
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