diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 59c7beb..e8312b0 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -39,6 +39,7 @@
 #include "xfs_quota.h"
 #include "xfs_dquot_item.h"
 #include "xfs_dquot.h"
+#include "xfs_reflink.h"
 
 
 #define XFS_WRITEIO_ALIGN(mp,off)	(((off) >> mp->m_writeio_log) \
@@ -961,8 +962,15 @@
 	if (XFS_FORCED_SHUTDOWN(mp))
 		return -EIO;
 
-	if ((flags & IOMAP_WRITE) &&
-	    !IS_DAX(inode) && !xfs_get_extsz_hint(ip)) {
+	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. */
 		return xfs_file_iomap_begin_delay(inode, offset, length, flags,
 				iomap);
 	}
diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index 7adbb83..5d796b7 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -51,6 +51,7 @@
 #include "xfs_btree.h"
 #include "xfs_bmap_btree.h"
 #include "xfs_reflink.h"
+#include "xfs_iomap.h"
 
 /*
  * Copy on Write of Shared Blocks
@@ -112,3 +113,218 @@
  * ioend structure.  Better yet, the more ground we can cover with one
  * ioend, the better.
  */
+
+/*
+ * Given an AG extent, find the lowest-numbered run of shared blocks
+ * within that range and return the range in fbno/flen.  If
+ * find_end_of_shared is true, return the longest contiguous extent of
+ * shared blocks.  If there are no shared extents, fbno and flen will
+ * be set to NULLAGBLOCK and 0, respectively.
+ */
+int
+xfs_reflink_find_shared(
+	struct xfs_mount	*mp,
+	xfs_agnumber_t		agno,
+	xfs_agblock_t		agbno,
+	xfs_extlen_t		aglen,
+	xfs_agblock_t		*fbno,
+	xfs_extlen_t		*flen,
+	bool			find_end_of_shared)
+{
+	struct xfs_buf		*agbp;
+	struct xfs_btree_cur	*cur;
+	int			error;
+
+	error = xfs_alloc_read_agf(mp, NULL, agno, 0, &agbp);
+	if (error)
+		return error;
+
+	cur = xfs_refcountbt_init_cursor(mp, NULL, agbp, agno, NULL);
+
+	error = xfs_refcount_find_shared(cur, agbno, aglen, fbno, flen,
+			find_end_of_shared);
+
+	xfs_btree_del_cursor(cur, error ? XFS_BTREE_ERROR : XFS_BTREE_NOERROR);
+
+	xfs_buf_relse(agbp);
+	return error;
+}
+
+/*
+ * Trim the mapping to the next block where there's a change in the
+ * shared/unshared status.  More specifically, this means that we
+ * find the lowest-numbered extent of shared blocks that coincides with
+ * the given block mapping.  If the shared extent overlaps the start of
+ * the mapping, trim the mapping to the end of the shared extent.  If
+ * the shared region intersects the mapping, trim the mapping to the
+ * start of the shared extent.  If there are no shared regions that
+ * overlap, just return the original extent.
+ */
+int
+xfs_reflink_trim_around_shared(
+	struct xfs_inode	*ip,
+	struct xfs_bmbt_irec	*irec,
+	bool			*shared,
+	bool			*trimmed)
+{
+	xfs_agnumber_t		agno;
+	xfs_agblock_t		agbno;
+	xfs_extlen_t		aglen;
+	xfs_agblock_t		fbno;
+	xfs_extlen_t		flen;
+	int			error = 0;
+
+	/* Holes, unwritten, and delalloc extents cannot be shared */
+	if (!xfs_is_reflink_inode(ip) ||
+	    ISUNWRITTEN(irec) ||
+	    irec->br_startblock == HOLESTARTBLOCK ||
+	    irec->br_startblock == DELAYSTARTBLOCK) {
+		*shared = false;
+		return 0;
+	}
+
+	trace_xfs_reflink_trim_around_shared(ip, irec);
+
+	agno = XFS_FSB_TO_AGNO(ip->i_mount, irec->br_startblock);
+	agbno = XFS_FSB_TO_AGBNO(ip->i_mount, irec->br_startblock);
+	aglen = irec->br_blockcount;
+
+	error = xfs_reflink_find_shared(ip->i_mount, agno, agbno,
+			aglen, &fbno, &flen, true);
+	if (error)
+		return error;
+
+	*shared = *trimmed = false;
+	if (fbno == NULLAGBLOCK) {
+		/* No shared blocks at all. */
+		return 0;
+	} else if (fbno == agbno) {
+		/*
+		 * The start of this extent is shared.  Truncate the
+		 * mapping at the end of the shared region so that a
+		 * subsequent iteration starts at the start of the
+		 * unshared region.
+		 */
+		irec->br_blockcount = flen;
+		*shared = true;
+		if (flen != aglen)
+			*trimmed = true;
+		return 0;
+	} else {
+		/*
+		 * There's a shared extent midway through this extent.
+		 * Truncate the mapping at the start of the shared
+		 * extent so that a subsequent iteration starts at the
+		 * start of the shared region.
+		 */
+		irec->br_blockcount = fbno - agbno;
+		*trimmed = true;
+		return 0;
+	}
+}
+
+/* Create a CoW reservation for a range of blocks within a file. */
+static int
+__xfs_reflink_reserve_cow(
+	struct xfs_inode	*ip,
+	xfs_fileoff_t		*offset_fsb,
+	xfs_fileoff_t		end_fsb)
+{
+	struct xfs_bmbt_irec	got, prev, imap;
+	xfs_fileoff_t		orig_end_fsb;
+	int			nimaps, eof = 0, error = 0;
+	bool			shared = false, trimmed = false;
+	xfs_extnum_t		idx;
+
+	/* Already reserved?  Skip the refcount btree access. */
+	xfs_bmap_search_extents(ip, *offset_fsb, 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;
+	}
+
+	/* 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);
+
+	/* Trim the mapping to the nearest shared extent boundary. */
+	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;
+
+	/* Not shared?  Just report the (potentially capped) extent. */
+	if (!shared)
+		goto done;
+
+	/*
+	 * Fork all the shared blocks from our write offset until the end of
+	 * the extent.
+	 */
+	error = xfs_qm_dqattach_locked(ip, 0);
+	if (error)
+		goto out_unlock;
+
+retry:
+	error = xfs_bmapi_reserve_delalloc(ip, XFS_COW_FORK, *offset_fsb,
+			end_fsb - *offset_fsb, &got,
+			&prev, &idx, eof);
+	switch (error) {
+	case 0:
+		break;
+	case -ENOSPC:
+	case -EDQUOT:
+		/* retry without any preallocation */
+		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;
+	}
+
+	trace_xfs_reflink_cow_alloc(ip, &got);
+done:
+	*offset_fsb = end_fsb;
+out_unlock:
+	return error;
+}
+
+/* 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;
+	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);
+		if (error) {
+			trace_xfs_reflink_reserve_cow_range_error(ip, error,
+				_RET_IP_);
+			break;
+		}
+	}
+	xfs_iunlock(ip, XFS_ILOCK_EXCL);
+
+	return error;
+}
diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
index 820b151..f824f87 100644
--- a/fs/xfs/xfs_reflink.h
+++ b/fs/xfs/xfs_reflink.h
@@ -20,4 +20,13 @@
 #ifndef __XFS_REFLINK_H
 #define __XFS_REFLINK_H 1
 
+extern int xfs_reflink_find_shared(struct xfs_mount *mp, xfs_agnumber_t agno,
+		xfs_agblock_t agbno, xfs_extlen_t aglen, xfs_agblock_t *fbno,
+		xfs_extlen_t *flen, bool find_maximal);
+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);
+
 #endif /* __XFS_REFLINK_H */