Re: [PATCH 4/4] xfs: allocate direct I/O COW blocks in iomap_begin

[Christoph Hellwig <hch@lst.de>]

On Thu, Feb 02, 2017 at 04:55:23PM -0800, Darrick J. Wong wrote:
> On Wed, Feb 01, 2017 at 10:42:39PM +0100, Christoph Hellwig wrote:
> > Instead of preallocating all the required COW blocks in the high-level
> > write code do it inside the iomap code, like we do for all other I/O.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/xfs/xfs_file.c    |   8 ---
> >  fs/xfs/xfs_iomap.c   |  31 +++++------
> >  fs/xfs/xfs_reflink.c | 147 ++++++++++++++++++++-------------------------------
> >  fs/xfs/xfs_reflink.h |   5 +-
> >  fs/xfs/xfs_trace.h   |   2 -
> >  5 files changed, 71 insertions(+), 122 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 06236bf..56ac5b7 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -559,14 +559,6 @@ xfs_file_dio_aio_write(
> >  	}
> >  
> >  	trace_xfs_file_direct_write(ip, count, iocb->ki_pos);
> > -
> > -	/* If this is a block-aligned directio CoW, remap immediately. */
> > -	if (xfs_is_reflink_inode(ip) && !unaligned_io) {
> > -		ret = xfs_reflink_allocate_cow_range(ip, iocb->ki_pos, count);
> > -		if (ret)
> > -			goto out;
> > -	}
> > -
> >  	ret = iomap_dio_rw(iocb, from, &xfs_iomap_ops, xfs_dio_write_end_io);
> >  out:
> >  	xfs_iunlock(ip, iolock);
> > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > index 9d811eb..de717e7 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -995,37 +995,31 @@ xfs_file_iomap_begin(
> >  	offset_fsb = XFS_B_TO_FSBT(mp, offset);
> >  	end_fsb = XFS_B_TO_FSB(mp, offset + length);
> >  
> > -	if (xfs_is_reflink_inode(ip) &&
> > -	    (flags & IOMAP_WRITE) && (flags & IOMAP_DIRECT)) {
> > -		shared = xfs_reflink_find_cow_mapping(ip, offset, &imap);
> > -		if (shared) {
> > -			xfs_iunlock(ip, lockmode);
> > -			goto alloc_done;
> > -		}
> > -		ASSERT(!isnullstartblock(imap.br_startblock));
> > -	}
> > -
> >  	error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
> >  			       &nimaps, 0);
> >  	if (error)
> >  		goto out_unlock;
> >  
> > -	if ((flags & IOMAP_REPORT) ||
> > -	    (xfs_is_reflink_inode(ip) &&
> > -	     (flags & IOMAP_WRITE) && (flags & IOMAP_DIRECT))) {
> > +	if (flags & IOMAP_REPORT) {
> >  		/* Trim the mapping to the nearest shared extent boundary. */
> >  		error = xfs_reflink_trim_around_shared(ip, &imap, &shared,
> >  				&trimmed);
> >  		if (error)
> >  			goto out_unlock;
> > -
> > -		ASSERT((flags & IOMAP_REPORT) || !shared);
> >  	}
> >  
> >  	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
> > -		error = xfs_reflink_reserve_cow(ip, &imap, &shared);
> > -		if (error)
> > -			goto out_unlock;
> > +		if (flags & IOMAP_DIRECT) {
> > +			/* may drop and re-acquire the ilock */
> > +			error = xfs_reflink_allocate_cow(ip, offset_fsb, end_fsb,
> > +					&imap, &shared, &lockmode);
> > +			if (error)
> > +				goto out_unlock;
> > +		} else {
> > +			error = xfs_reflink_reserve_cow(ip, &imap, &shared);
> > +			if (error)
> > +				goto out_unlock;
> > +		}
> >  
> >  		end_fsb = imap.br_startoff + imap.br_blockcount;
> >  		length = XFS_FSB_TO_B(mp, end_fsb) - offset;
> > @@ -1054,7 +1048,6 @@ xfs_file_iomap_begin(
> >  		if (error)
> >  			return error;
> >  
> > -alloc_done:
> >  		iomap->flags = IOMAP_F_NEW;
> >  		trace_xfs_iomap_alloc(ip, offset, length, 0, &imap);
> >  	} else {
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > index 1a96741..d5a2cf2 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -384,74 +384,84 @@ xfs_reflink_convert_cow(
> >  }
> >  
> >  /* Allocate all CoW reservations covering a range of blocks in a file. */
> > -static int
> > -__xfs_reflink_allocate_cow(
> > +int
> > +xfs_reflink_allocate_cow(
> >  	struct xfs_inode	*ip,
> > -	xfs_fileoff_t		*offset_fsb,
> > -	xfs_fileoff_t		end_fsb)
> > +	xfs_fileoff_t		offset_fsb,
> > +	xfs_fileoff_t		end_fsb,
> > +	struct xfs_bmbt_irec	*imap,
> > +	bool			*shared,
> > +	uint			*lockmode)
> >  {
> >  	struct xfs_mount	*mp = ip->i_mount;
> > -	struct xfs_bmbt_irec	imap, got;
> > +	struct xfs_bmbt_irec	got;
> >  	struct xfs_defer_ops	dfops;
> > -	struct xfs_trans	*tp;
> > +	struct xfs_trans	*tp = NULL;
> >  	xfs_fsblock_t		first_block;
> > -	int			nimaps, error, lockmode;
> > -	bool			shared, trimmed;
> > +	int			nimaps, error = 0;
> > +	bool			trimmed;
> >  	xfs_filblks_t		resaligned;
> > -	xfs_extlen_t		resblks;
> > +	xfs_extlen_t		resblks = 0;
> >  	xfs_extnum_t		idx;
> >  
> > -	resaligned = xfs_aligned_fsb_count(*offset_fsb, end_fsb - *offset_fsb,
> > -			xfs_get_cowextsz_hint(ip));
> > -	resblks = XFS_DIOSTRAT_SPACE_RES(mp, resaligned);
> > -
> > -	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0, 0, &tp);
> > -	if (error)
> > -		return error;
> > -
> > -	lockmode = XFS_ILOCK_EXCL;
> > -	xfs_ilock(ip, lockmode);
> > +retry:
> > +	ASSERT(xfs_is_reflink_inode(ip));
> > +	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL | XFS_ILOCK_SHARED));
> >  
> >  	/*
> >  	 * Even if the extent is not shared we might have a preallocation for
> >  	 * it in the COW fork.  If so use it.
> >  	 */
> > -	if (xfs_iext_lookup_extent(ip, ip->i_cowfp, *offset_fsb, &idx, &got) &&
> > -	    got.br_startoff <= *offset_fsb) {
> > +	if (xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &idx, &got) &&
> > +	    got.br_startoff <= offset_fsb) {
> > +		*shared = true;
> > +
> >  		/* If we have a real allocation in the COW fork we're done. */
> >  		if (!isnullstartblock(got.br_startblock)) {
> > -			xfs_trim_extent(&got, *offset_fsb,
> > -					end_fsb - *offset_fsb);
> > -			*offset_fsb = got.br_startoff + got.br_blockcount;
> > -			goto out_trans_cancel;
> > +			xfs_trim_extent(&got, offset_fsb, end_fsb - offset_fsb);
> > +			*imap = got;
> > +			goto out;
> >  		}
> > +
> > +		xfs_trim_extent(imap, got.br_startoff, got.br_blockcount);
> >  	} else {
> > -		nimaps = 1;
> > -		error = xfs_bmapi_read(ip, *offset_fsb, end_fsb - *offset_fsb,
> > -				&imap, &nimaps, 0);
> > +		error = xfs_reflink_trim_around_shared(ip, imap, shared, &trimmed);
> > +		if (error || !*shared)
> > +			goto out;
> > +	}
> > +
> > +	if (!tp) {
> > +		resaligned = xfs_aligned_fsb_count(offset_fsb,
> > +			end_fsb - offset_fsb, xfs_get_cowextsz_hint(ip));
> > +		resblks = XFS_DIOSTRAT_SPACE_RES(mp, resaligned);
> > +
> > +		xfs_iunlock(ip, *lockmode);
> > +		error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0, 0, &tp);
> > +		*lockmode = XFS_ILOCK_EXCL;
> > +		xfs_ilock(ip, *lockmode);
> > +
> >  		if (error)
> > -			goto out_trans_cancel;
> > -		ASSERT(nimaps == 1);
> > +			return error;
> >  
> > -		/* Trim the mapping to the nearest shared extent boundary. */
> > -		error = xfs_reflink_trim_around_shared(ip, &imap, &shared,
> > -				&trimmed);
> > +		error = xfs_qm_dqattach_locked(ip, 0);
> >  		if (error)
> > -			goto out_trans_cancel;
> > +			goto out;
> >  
> > -		if (!shared) {
> > -			*offset_fsb = imap.br_startoff + imap.br_blockcount;
> > -			goto out_trans_cancel;
> > -		}
> > +		/* Read extent from the source file. */
> > +		nimaps = 1;
> > +		error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb,
> > +				imap, &nimaps, 0);
> > +		if (error)
> > +			goto out;
> > +		ASSERT(nimaps == 1);
> >  
> > -		*offset_fsb = imap.br_startoff;
> > -		end_fsb = imap.br_startoff + imap.br_blockcount;
> > +		goto retry;
> >  	}
> >  
> >  	error = xfs_trans_reserve_quota_nblks(tp, ip, resblks, 0,
> >  			XFS_QMOPT_RES_REGBLKS);
> >  	if (error)
> > -		goto out_trans_cancel;
> > +		goto out;
> >  
> >  	xfs_trans_ijoin(tp, ip, 0);
> >  
> > @@ -459,9 +469,9 @@ __xfs_reflink_allocate_cow(
> >  	nimaps = 1;
> >  
> >  	/* Allocate the entire reservation as unwritten blocks. */
> > -	error = xfs_bmapi_write(tp, ip, *offset_fsb, end_fsb - *offset_fsb,
> > +	error = xfs_bmapi_write(tp, ip, imap->br_startoff, imap->br_blockcount,
> >  			XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC, &first_block,
> > -			resblks, &imap, &nimaps, &dfops);
> > +			resblks, imap, &nimaps, &dfops);
> >  	if (error)
> >  		goto out_bmap_cancel;
> >  
> > @@ -470,60 +480,15 @@ __xfs_reflink_allocate_cow(
> >  	if (error)
> >  		goto out_bmap_cancel;
> >  
> > -	error = xfs_trans_commit(tp);
> > -	if (error)
> > -		goto out_unlock;
> > -
> > -	*offset_fsb = imap.br_startoff + imap.br_blockcount;
> > -
> > -out_unlock:
> > -	xfs_iunlock(ip, lockmode);
> > -	return error;
> > +	return xfs_trans_commit(tp);
> >  
> >  out_bmap_cancel:
> >  	xfs_defer_cancel(&dfops);
> >  	xfs_trans_unreserve_quota_nblks(tp, ip, (long)resblks, 0,
> >  			XFS_QMOPT_RES_REGBLKS);
> > -out_trans_cancel:
> > -	xfs_trans_cancel(tp);
> > -	goto out_unlock;
> > -}
> > -
> > -/* Allocate all CoW reservations covering a part of a file. */
> > -int
> > -xfs_reflink_allocate_cow_range(
> > -	struct xfs_inode	*ip,
> > -	xfs_off_t		offset,
> > -	xfs_off_t		count)
> > -{
> > -	struct xfs_mount	*mp = ip->i_mount;
> > -	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
> > -	xfs_fileoff_t		end_fsb = XFS_B_TO_FSB(mp, offset + count);
> > -	int			error;
> > -
> > -	ASSERT(xfs_is_reflink_inode(ip));
> > -
> > -	trace_xfs_reflink_allocate_cow_range(ip, offset, count);
> > -
> > -	/*
> > -	 * Make sure that the dquots are there.
> > -	 */
> > -	error = xfs_qm_dqattach(ip, 0);
> > -	if (error)
> > -		return error;
> > -
> > -	while (offset_fsb < end_fsb) {
> > -		error = __xfs_reflink_allocate_cow(ip, &offset_fsb, end_fsb);
> > -		if (error) {
> > -			trace_xfs_reflink_allocate_cow_range_error(ip, error,
> > -					_RET_IP_);
> > -			goto out;
> > -		}
> > -	}
> > -
> > -	/* Convert the CoW extents to regular. */
> > -	error = xfs_reflink_convert_cow(ip, offset, count);
> 
> I think this is incorrect -- we create an unwritten extent in the cow
> fork above (BMAP_COWFORK | BMAP_PREALLOC) but we no longer convert the
> part we're actually writing to a real extent.  This means that
> _reflink_cow won't actually remap anything that we've written.

It's not correct and I messed up when rebasing - the version I tested
still had it and worked.  Sigh..