diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 1dabf2e..436e109 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -566,6 +566,17 @@ xfs_file_iomap_begin_delay(
 	xfs_bmap_search_extents(ip, offset_fsb, XFS_DATA_FORK, &eof, &idx,
 			&got, &prev);
 	if (!eof && got.br_startoff <= offset_fsb) {
+		if (xfs_is_reflink_inode(ip)) {
+			bool		shared;
+
+			end_fsb = min(XFS_B_TO_FSB(mp, offset + count),
+					maxbytes_fsb);
+			xfs_trim_extent(&got, offset_fsb, end_fsb - offset_fsb);
+			error = xfs_reflink_reserve_cow(ip, &got, &shared);
+			if (error)
+				goto out_unlock;
+		}
+
 		trace_xfs_iomap_found(ip, offset, count, 0, &got);
 		goto done;
 	}
@@ -961,19 +972,13 @@ xfs_file_iomap_begin(
 	struct xfs_mount	*mp = ip->i_mount;
 	struct xfs_bmbt_irec	imap;
 	xfs_fileoff_t		offset_fsb, end_fsb;
-	bool			shared, trimmed;
 	int			nimaps = 1, error = 0;
+	bool			shared = false, trimmed = false;
 	unsigned		lockmode;
 
 	if (XFS_FORCED_SHUTDOWN(mp))
 		return -EIO;
 
-	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
-		error = xfs_reflink_reserve_cow_range(ip, offset, length);
-		if (error < 0)
-			return error;
-	}
-
 	if ((flags & IOMAP_WRITE) && !IS_DAX(inode) &&
 		   !xfs_get_extsz_hint(ip)) {
 		/* Reserve delalloc blocks for regular writeback. */
@@ -981,7 +986,16 @@ xfs_file_iomap_begin(
 				iomap);
 	}
 
-	lockmode = xfs_ilock_data_map_shared(ip);
+	/*
+	 * COW writes will allocate delalloc space, so we need to make sure
+	 * to take the lock exclusively here.
+	 */
+	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
+		lockmode = XFS_ILOCK_EXCL;
+		xfs_ilock(ip, XFS_ILOCK_EXCL);
+	} else {
+		lockmode = xfs_ilock_data_map_shared(ip);
+	}
 
 	ASSERT(offset <= mp->m_super->s_maxbytes);
 	if ((xfs_fsize_t)offset + length > mp->m_super->s_maxbytes)
@@ -991,19 +1005,24 @@ xfs_file_iomap_begin(
 
 	error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
 			       &nimaps, 0);
-	if (error) {
-		xfs_iunlock(ip, lockmode);
-		return error;
-	}
+	if (error)
+		goto out_unlock;
 
-	if (flags & (IOMAP_WRITE | IOMAP_ZERO | IOMAP_REPORT)) {
+	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) {
-			xfs_iunlock(ip, lockmode);
-			return error;
-		}
+		if (error)
+			goto out_unlock;
+	}
+
+	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
+		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;
 	}
 
 	if ((flags & IOMAP_WRITE) && imap_needs_alloc(inode, &imap, nimaps)) {
@@ -1042,6 +1061,9 @@ xfs_file_iomap_begin(
 	if (shared)
 		iomap->flags |= IOMAP_F_SHARED;
 	return 0;
+out_unlock:
+	xfs_iunlock(ip, lockmode);
+	return error;
 }
 
 static int
diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index 46c824c..5d230ea 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -228,50 +228,54 @@ xfs_reflink_trim_around_shared(
 	}
 }
 
-/* Create a CoW reservation for a range of blocks within a file. */
-static int
-__xfs_reflink_reserve_cow(
+/*
+ * Trim the passed in imap to the next shared/unshared extent boundary, and
+ * if imap->br_startoff points to a shared extent reserve space for it in the
+ * COW fork.  In this case *shared is set to true, else to false.
+ *
+ * Note that imap will always contain the block numbers for the existing blocks
+ * in the data fork, as the upper layers need them for read-modify-write
+ * operations.
+ */
+int
+xfs_reflink_reserve_cow(
 	struct xfs_inode	*ip,
-	xfs_fileoff_t		*offset_fsb,
-	xfs_fileoff_t		end_fsb,
-	bool			*skipped)
+	struct xfs_bmbt_irec	*imap,
+	bool			*shared)
 {
-	struct xfs_bmbt_irec	got, prev, imap;
-	xfs_fileoff_t		orig_end_fsb;
-	int			nimaps, eof = 0, error = 0;
-	bool			shared = false, trimmed = false;
+	struct xfs_bmbt_irec	got, prev;
+	xfs_fileoff_t		end_fsb, orig_end_fsb;
+	int			eof = 0, error = 0;
+	bool			trimmed;
 	xfs_extnum_t		idx;
 	xfs_extlen_t		align;
 
-	/* Already reserved?  Skip the refcount btree access. */
-	xfs_bmap_search_extents(ip, *offset_fsb, XFS_COW_FORK, &eof, &idx,
+	/*
+	 * Search the COW fork extent list first.  This serves two purposes:
+	 * first this implement the speculative preallocation using cowextisze,
+	 * so that we also unshared block adjacent to shared blocks instead
+	 * of just the shared blocks themselves.  Second the lookup in the
+	 * extent list is generally faster than going out to the shared extent
+	 * tree.
+	 */
+	xfs_bmap_search_extents(ip, imap->br_startoff, XFS_COW_FORK, &eof, &idx,
 			&got, &prev);
-	if (!eof && got.br_startoff <= *offset_fsb) {
-		end_fsb = orig_end_fsb = got.br_startoff + got.br_blockcount;
-		trace_xfs_reflink_cow_found(ip, &got);
-		goto done;
-	}
+	if (!eof && got.br_startoff <= imap->br_startoff) {
+		trace_xfs_reflink_cow_found(ip, imap);
+		xfs_trim_extent(imap, got.br_startoff, got.br_blockcount);
 
-	/* 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_unlock;
-	ASSERT(nimaps == 1);
+		*shared = true;
+		return 0;
+	}
 
 	/* Trim the mapping to the nearest shared extent boundary. */
-	error = xfs_reflink_trim_around_shared(ip, &imap, &shared, &trimmed);
+	error = xfs_reflink_trim_around_shared(ip, imap, shared, &trimmed);
 	if (error)
-		goto out_unlock;
-
-	end_fsb = orig_end_fsb = imap.br_startoff + imap.br_blockcount;
+		return error;
 
 	/* Not shared?  Just report the (potentially capped) extent. */
-	if (!shared) {
-		*skipped = true;
-		goto done;
-	}
+	if (!*shared)
+		return 0;
 
 	/*
 	 * Fork all the shared blocks from our write offset until the end of
@@ -279,73 +283,40 @@ __xfs_reflink_reserve_cow(
 	 */
 	error = xfs_qm_dqattach_locked(ip, 0);
 	if (error)
-		goto out_unlock;
+		return error;
+
+	end_fsb = orig_end_fsb = imap->br_startoff + imap->br_blockcount;
 
 	align = xfs_eof_alignment(ip, xfs_get_cowextsz_hint(ip));
 	if (align)
 		end_fsb = roundup_64(end_fsb, align);
 
 retry:
-	error = xfs_bmapi_reserve_delalloc(ip, XFS_COW_FORK, *offset_fsb,
-			end_fsb - *offset_fsb, &got,
-			&prev, &idx, eof);
+	error = xfs_bmapi_reserve_delalloc(ip, XFS_COW_FORK, imap->br_startoff,
+			end_fsb - imap->br_startoff, &got, &prev, &idx, eof);
 	switch (error) {
 	case 0:
 		break;
 	case -ENOSPC:
 	case -EDQUOT:
 		/* retry without any preallocation */
-		trace_xfs_reflink_cow_enospc(ip, &imap);
+		trace_xfs_reflink_cow_enospc(ip, imap);
 		if (end_fsb != orig_end_fsb) {
 			end_fsb = orig_end_fsb;
 			goto retry;
 		}
 		/*FALLTHRU*/
 	default:
-		goto out_unlock;
+		return error;
 	}
 
 	if (end_fsb != orig_end_fsb)
 		xfs_inode_set_cowblocks_tag(ip);
 
 	trace_xfs_reflink_cow_alloc(ip, &got);
-done:
-	*offset_fsb = end_fsb;
-out_unlock:
-	return error;
+	return 0;
 }
 
-/* Create a CoW reservation for part of a file. */
-int
-xfs_reflink_reserve_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, end_fsb;
-	bool			skipped = false;
-	int			error;
-
-	trace_xfs_reflink_reserve_cow_range(ip, offset, count);
-
-	offset_fsb = XFS_B_TO_FSBT(mp, offset);
-	end_fsb = XFS_B_TO_FSB(mp, offset + count);
-
-	xfs_ilock(ip, XFS_ILOCK_EXCL);
-	while (offset_fsb < end_fsb) {
-		error = __xfs_reflink_reserve_cow(ip, &offset_fsb, end_fsb,
-				&skipped);
-		if (error) {
-			trace_xfs_reflink_reserve_cow_range_error(ip, error,
-				_RET_IP_);
-			break;
-		}
-	}
-	xfs_iunlock(ip, XFS_ILOCK_EXCL);
-
-	return error;
-}
 
 /* Allocate all CoW reservations covering a range of blocks in a file. */
 static int
@@ -359,9 +330,8 @@ __xfs_reflink_allocate_cow(
 	struct xfs_defer_ops	dfops;
 	struct xfs_trans	*tp;
 	xfs_fsblock_t		first_block;
-	xfs_fileoff_t		next_fsb;
 	int			nimaps = 1, error;
-	bool			skipped = false;
+	bool			shared;
 
 	xfs_defer_init(&dfops, &first_block);
 
@@ -372,33 +342,38 @@ __xfs_reflink_allocate_cow(
 
 	xfs_ilock(ip, XFS_ILOCK_EXCL);
 
-	next_fsb = *offset_fsb;
-	error = __xfs_reflink_reserve_cow(ip, &next_fsb, end_fsb, &skipped);
+	/* 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_unlock;
+	ASSERT(nimaps == 1);
+
+	error = xfs_reflink_reserve_cow(ip, &imap, &shared);
 	if (error)
 		goto out_trans_cancel;
 
-	if (skipped) {
-		*offset_fsb = next_fsb;
+	if (!shared) {
+		*offset_fsb = imap.br_startoff + imap.br_blockcount;
 		goto out_trans_cancel;
 	}
 
 	xfs_trans_ijoin(tp, ip, 0);
-	error = xfs_bmapi_write(tp, ip, *offset_fsb, next_fsb - *offset_fsb,
+	error = xfs_bmapi_write(tp, ip, imap.br_startoff, imap.br_blockcount,
 			XFS_BMAPI_COWFORK, &first_block,
 			XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK),
 			&imap, &nimaps, &dfops);
 	if (error)
 		goto out_trans_cancel;
 
-	/* We might not have been able to map the whole delalloc extent */
-	*offset_fsb = min(*offset_fsb + imap.br_blockcount, next_fsb);
-
 	error = xfs_defer_finish(&tp, &dfops, NULL);
 	if (error)
 		goto out_trans_cancel;
 
 	error = xfs_trans_commit(tp);
 
+	*offset_fsb = imap.br_startoff + imap.br_blockcount;
 out_unlock:
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 	return error;
diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
index 5dc3c8a..9f76924 100644
--- a/fs/xfs/xfs_reflink.h
+++ b/fs/xfs/xfs_reflink.h
@@ -26,8 +26,8 @@ extern int xfs_reflink_find_shared(struct xfs_mount *mp, xfs_agnumber_t agno,
 extern int xfs_reflink_trim_around_shared(struct xfs_inode *ip,
 		struct xfs_bmbt_irec *irec, bool *shared, bool *trimmed);
 
-extern int xfs_reflink_reserve_cow_range(struct xfs_inode *ip,
-		xfs_off_t offset, xfs_off_t count);
+extern int xfs_reflink_reserve_cow(struct xfs_inode *ip,
+		struct xfs_bmbt_irec *imap, bool *shared);
 extern int xfs_reflink_allocate_cow_range(struct xfs_inode *ip,
 		xfs_off_t offset, xfs_off_t count);
 extern bool xfs_reflink_find_cow_mapping(struct xfs_inode *ip, xfs_off_t offset,
diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
index ad188d3..72f9f6b 100644
--- a/fs/xfs/xfs_trace.h
+++ b/fs/xfs/xfs_trace.h
@@ -3346,7 +3346,7 @@ DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_alloc);
 DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_found);
 DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_enospc);
 
-DEFINE_RW_EVENT(xfs_reflink_reserve_cow_range);
+DEFINE_RW_EVENT(xfs_reflink_reserve_cow);
 DEFINE_RW_EVENT(xfs_reflink_allocate_cow_range);
 
 DEFINE_INODE_IREC_EVENT(xfs_reflink_bounce_dio_write);
@@ -3358,7 +3358,6 @@ DEFINE_SIMPLE_IO_EVENT(xfs_reflink_end_cow);
 DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_remap);
 DEFINE_INODE_IREC_EVENT(xfs_reflink_cow_remap_piece);
 
-DEFINE_INODE_ERROR_EVENT(xfs_reflink_reserve_cow_range_error);
 DEFINE_INODE_ERROR_EVENT(xfs_reflink_allocate_cow_range_error);
 DEFINE_INODE_ERROR_EVENT(xfs_reflink_cancel_cow_range_error);
 DEFINE_INODE_ERROR_EVENT(xfs_reflink_end_cow_error);
