diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index e4bf2c4..9702f4c 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -3666,7 +3666,9 @@
 	else if (mp->m_dalign)
 		stripe_align = mp->m_dalign;
 
-	if (xfs_alloc_is_userdata(ap->datatype))
+	if (ap->flags & XFS_BMAPI_COWFORK)
+		align = xfs_get_cowextsz_hint(ap->ip);
+	else if (xfs_alloc_is_userdata(ap->datatype))
 		align = xfs_get_extsz_hint(ap->ip);
 	if (unlikely(align)) {
 		error = xfs_bmap_extsize_align(mp, &ap->got, &ap->prev,
@@ -4192,7 +4194,10 @@
 		alen = XFS_FILBLKS_MIN(alen, got->br_startoff - aoff);
 
 	/* Figure out the extent size, adjust alen */
-	extsz = xfs_get_extsz_hint(ip);
+	if (whichfork == XFS_COW_FORK)
+		extsz = xfs_get_cowextsz_hint(ip);
+	else
+		extsz = xfs_get_extsz_hint(ip);
 	if (extsz) {
 		error = xfs_bmap_extsize_align(mp, got, prev, extsz, rt, eof,
 					       1, 0, &aoff, &alen);
diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
index 777fa97..5e04ea2 100644
--- a/fs/xfs/libxfs/xfs_format.h
+++ b/fs/xfs/libxfs/xfs_format.h
@@ -901,7 +901,8 @@
 	__be64		di_changecount;	/* number of attribute changes */
 	__be64		di_lsn;		/* flush sequence */
 	__be64		di_flags2;	/* more random flags */
-	__u8		di_pad2[16];	/* more padding for future expansion */
+	__be32		di_cowextsize;	/* basic cow extent size for file */
+	__u8		di_pad2[12];	/* more padding for future expansion */
 
 	/* fields only written to during inode creation */
 	xfs_timestamp_t	di_crtime;	/* time created */
diff --git a/fs/xfs/libxfs/xfs_fs.h b/fs/xfs/libxfs/xfs_fs.h
index 3d1efe5..b72dc82 100644
--- a/fs/xfs/libxfs/xfs_fs.h
+++ b/fs/xfs/libxfs/xfs_fs.h
@@ -278,7 +278,8 @@
 #define	bs_projid	bs_projid_lo	/* (previously just bs_projid)	*/
 	__u16		bs_forkoff;	/* inode fork offset in bytes	*/
 	__u16		bs_projid_hi;	/* higher part of project id	*/
-	unsigned char	bs_pad[10];	/* pad space, unused		*/
+	unsigned char	bs_pad[6];	/* pad space, unused		*/
+	__u32		bs_cowextsize;	/* cow extent size		*/
 	__u32		bs_dmevmask;	/* DMIG event mask		*/
 	__u16		bs_dmstate;	/* DMIG state info		*/
 	__u16		bs_aextents;	/* attribute number of extents	*/
diff --git a/fs/xfs/libxfs/xfs_inode_buf.c b/fs/xfs/libxfs/xfs_inode_buf.c
index 4b9769e..a3e8038 100644
--- a/fs/xfs/libxfs/xfs_inode_buf.c
+++ b/fs/xfs/libxfs/xfs_inode_buf.c
@@ -256,6 +256,7 @@
 		to->di_crtime.t_sec = be32_to_cpu(from->di_crtime.t_sec);
 		to->di_crtime.t_nsec = be32_to_cpu(from->di_crtime.t_nsec);
 		to->di_flags2 = be64_to_cpu(from->di_flags2);
+		to->di_cowextsize = be32_to_cpu(from->di_cowextsize);
 	}
 }
 
@@ -305,7 +306,7 @@
 		to->di_crtime.t_sec = cpu_to_be32(from->di_crtime.t_sec);
 		to->di_crtime.t_nsec = cpu_to_be32(from->di_crtime.t_nsec);
 		to->di_flags2 = cpu_to_be64(from->di_flags2);
-
+		to->di_cowextsize = cpu_to_be32(from->di_cowextsize);
 		to->di_ino = cpu_to_be64(ip->i_ino);
 		to->di_lsn = cpu_to_be64(lsn);
 		memset(to->di_pad2, 0, sizeof(to->di_pad2));
@@ -357,6 +358,7 @@
 		to->di_crtime.t_sec = cpu_to_be32(from->di_crtime.t_sec);
 		to->di_crtime.t_nsec = cpu_to_be32(from->di_crtime.t_nsec);
 		to->di_flags2 = cpu_to_be64(from->di_flags2);
+		to->di_cowextsize = cpu_to_be32(from->di_cowextsize);
 		to->di_ino = cpu_to_be64(from->di_ino);
 		to->di_lsn = cpu_to_be64(from->di_lsn);
 		memcpy(to->di_pad2, from->di_pad2, sizeof(to->di_pad2));
diff --git a/fs/xfs/libxfs/xfs_inode_buf.h b/fs/xfs/libxfs/xfs_inode_buf.h
index 7c4dd32..62d9d46 100644
--- a/fs/xfs/libxfs/xfs_inode_buf.h
+++ b/fs/xfs/libxfs/xfs_inode_buf.h
@@ -47,6 +47,7 @@
 	__uint16_t	di_flags;	/* random flags, XFS_DIFLAG_... */
 
 	__uint64_t	di_flags2;	/* more random flags */
+	__uint32_t	di_cowextsize;	/* basic cow extent size for file */
 
 	xfs_ictimestamp_t di_crtime;	/* time created */
 };
diff --git a/fs/xfs/libxfs/xfs_log_format.h b/fs/xfs/libxfs/xfs_log_format.h
index 5dd7c2e..364ce6f 100644
--- a/fs/xfs/libxfs/xfs_log_format.h
+++ b/fs/xfs/libxfs/xfs_log_format.h
@@ -423,7 +423,8 @@
 	__uint64_t	di_changecount;	/* number of attribute changes */
 	xfs_lsn_t	di_lsn;		/* flush sequence */
 	__uint64_t	di_flags2;	/* more random flags */
-	__uint8_t	di_pad2[16];	/* more padding for future expansion */
+	__uint32_t	di_cowextsize;	/* basic cow extent size for file */
+	__uint8_t	di_pad2[12];	/* more padding for future expansion */
 
 	/* fields only written to during inode creation */
 	xfs_ictimestamp_t di_crtime;	/* time created */
diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index e2f46e6..4a807f0 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -582,8 +582,13 @@
 		if (ip->i_cformat != XFS_DINODE_FMT_EXTENTS)
 			return -EINVAL;
 
-		prealloced = 0;
-		fixlen = XFS_ISIZE(ip);
+		if (xfs_get_cowextsz_hint(ip)) {
+			prealloced = 1;
+			fixlen = mp->m_super->s_maxbytes;
+		} else {
+			prealloced = 0;
+			fixlen = XFS_ISIZE(ip);
+		}
 		break;
 	default:
 		if (ip->i_d.di_format != XFS_DINODE_FMT_EXTENTS &&
diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index f072f48..09640bd 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -78,6 +78,27 @@
 }
 
 /*
+ * Helper function to extract CoW extent size hint from inode.
+ * Between the extent size hint and the CoW extent size hint, we
+ * return the greater of the two.
+ */
+xfs_extlen_t
+xfs_get_cowextsz_hint(
+	struct xfs_inode	*ip)
+{
+	xfs_extlen_t		a, b;
+
+	a = 0;
+	if (ip->i_d.di_flags2 & XFS_DIFLAG2_COWEXTSIZE)
+		a = ip->i_d.di_cowextsize;
+	b = xfs_get_extsz_hint(ip);
+
+	if (a > b)
+		return a;
+	return b;
+}
+
+/*
  * These two are wrapper routines around the xfs_ilock() routine used to
  * centralize some grungy code.  They are used in places that wish to lock the
  * inode solely for reading the extents.  The reason these places can't just
@@ -652,6 +673,8 @@
 	if (di_flags2 & XFS_DIFLAG2_ANY) {
 		if (di_flags2 & XFS_DIFLAG2_DAX)
 			flags |= FS_XFLAG_DAX;
+		if (di_flags2 & XFS_DIFLAG2_COWEXTSIZE)
+			flags |= FS_XFLAG_COWEXTSIZE;
 	}
 
 	if (has_attr)
@@ -835,6 +858,7 @@
 	if (ip->i_d.di_version == 3) {
 		inode->i_version = 1;
 		ip->i_d.di_flags2 = 0;
+		ip->i_d.di_cowextsize = 0;
 		ip->i_d.di_crtime.t_sec = (__int32_t)tv.tv_sec;
 		ip->i_d.di_crtime.t_nsec = (__int32_t)tv.tv_nsec;
 	}
@@ -897,6 +921,15 @@
 			ip->i_d.di_flags |= di_flags;
 			ip->i_d.di_flags2 |= di_flags2;
 		}
+		if (pip &&
+		    (pip->i_d.di_flags2 & XFS_DIFLAG2_ANY) &&
+		    pip->i_d.di_version == 3 &&
+		    ip->i_d.di_version == 3) {
+			if (pip->i_d.di_flags2 & XFS_DIFLAG2_COWEXTSIZE) {
+				ip->i_d.di_flags2 |= XFS_DIFLAG2_COWEXTSIZE;
+				ip->i_d.di_cowextsize = pip->i_d.di_cowextsize;
+			}
+		}
 		/* FALLTHROUGH */
 	case S_IFLNK:
 		ip->i_d.di_format = XFS_DINODE_FMT_EXTENTS;
diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
index ff6fc03..6d63dc0 100644
--- a/fs/xfs/xfs_inode.h
+++ b/fs/xfs/xfs_inode.h
@@ -426,6 +426,7 @@
 void		xfs_lock_two_inodes(xfs_inode_t *, xfs_inode_t *, uint);
 
 xfs_extlen_t	xfs_get_extsz_hint(struct xfs_inode *ip);
+xfs_extlen_t	xfs_get_cowextsz_hint(struct xfs_inode *ip);
 
 int		xfs_dir_ialloc(struct xfs_trans **, struct xfs_inode *, umode_t,
 			       xfs_nlink_t, xfs_dev_t, prid_t, int,
diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
index 892c2ac..9610e9c 100644
--- a/fs/xfs/xfs_inode_item.c
+++ b/fs/xfs/xfs_inode_item.c
@@ -368,7 +368,7 @@
 		to->di_crtime.t_sec = from->di_crtime.t_sec;
 		to->di_crtime.t_nsec = from->di_crtime.t_nsec;
 		to->di_flags2 = from->di_flags2;
-
+		to->di_cowextsize = from->di_cowextsize;
 		to->di_ino = ip->i_ino;
 		to->di_lsn = lsn;
 		memset(to->di_pad2, 0, sizeof(to->di_pad2));
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 96a70fd..1388a127 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -903,6 +903,8 @@
 	xfs_ilock(ip, XFS_ILOCK_SHARED);
 	fa.fsx_xflags = xfs_ip2xflags(ip);
 	fa.fsx_extsize = ip->i_d.di_extsize << ip->i_mount->m_sb.sb_blocklog;
+	fa.fsx_cowextsize = ip->i_d.di_cowextsize <<
+			ip->i_mount->m_sb.sb_blocklog;
 	fa.fsx_projid = xfs_get_projid(ip);
 
 	if (attr) {
@@ -973,12 +975,13 @@
 	if (ip->i_d.di_version < 3)
 		return;
 
-	di_flags2 = 0;
+	di_flags2 = (ip->i_d.di_flags2 & XFS_DIFLAG2_REFLINK);
 	if (xflags & FS_XFLAG_DAX)
 		di_flags2 |= XFS_DIFLAG2_DAX;
+	if (xflags & FS_XFLAG_COWEXTSIZE)
+		di_flags2 |= XFS_DIFLAG2_COWEXTSIZE;
 
 	ip->i_d.di_flags2 = di_flags2;
-
 }
 
 STATIC void
@@ -1219,6 +1222,56 @@
 	return 0;
 }
 
+/*
+ * CoW extent size hint validation rules are:
+ *
+ * 1. CoW extent size hint can only be set if reflink is enabled on the fs.
+ *    The inode does not have to have any shared blocks, but it must be a v3.
+ * 2. FS_XFLAG_COWEXTSIZE is only valid for directories and regular files;
+ *    for a directory, the hint is propagated to new files.
+ * 3. Can be changed on files & directories at any time.
+ * 4. CoW extsize hint of 0 turns off hints, clears inode flags.
+ * 5. Extent size must be a multiple of the appropriate block size.
+ * 6. The extent size hint must be limited to half the AG size to avoid
+ *    alignment extending the extent beyond the limits of the AG.
+ */
+static int
+xfs_ioctl_setattr_check_cowextsize(
+	struct xfs_inode	*ip,
+	struct fsxattr		*fa)
+{
+	struct xfs_mount	*mp = ip->i_mount;
+
+	if (!(fa->fsx_xflags & FS_XFLAG_COWEXTSIZE))
+		return 0;
+
+	if (!xfs_sb_version_hasreflink(&ip->i_mount->m_sb) ||
+	    ip->i_d.di_version != 3)
+		return -EINVAL;
+
+	if (!S_ISREG(VFS_I(ip)->i_mode) && !S_ISDIR(VFS_I(ip)->i_mode))
+		return -EINVAL;
+
+	if (fa->fsx_cowextsize != 0) {
+		xfs_extlen_t    size;
+		xfs_fsblock_t   cowextsize_fsb;
+
+		cowextsize_fsb = XFS_B_TO_FSB(mp, fa->fsx_cowextsize);
+		if (cowextsize_fsb > MAXEXTLEN)
+			return -EINVAL;
+
+		size = mp->m_sb.sb_blocksize;
+		if (cowextsize_fsb > mp->m_sb.sb_agblocks / 2)
+			return -EINVAL;
+
+		if (fa->fsx_cowextsize % size)
+			return -EINVAL;
+	} else
+		fa->fsx_xflags &= ~FS_XFLAG_COWEXTSIZE;
+
+	return 0;
+}
+
 static int
 xfs_ioctl_setattr_check_projid(
 	struct xfs_inode	*ip,
@@ -1311,6 +1364,10 @@
 	if (code)
 		goto error_trans_cancel;
 
+	code = xfs_ioctl_setattr_check_cowextsize(ip, fa);
+	if (code)
+		goto error_trans_cancel;
+
 	code = xfs_ioctl_setattr_xflags(tp, ip, fa);
 	if (code)
 		goto error_trans_cancel;
@@ -1346,6 +1403,12 @@
 		ip->i_d.di_extsize = fa->fsx_extsize >> mp->m_sb.sb_blocklog;
 	else
 		ip->i_d.di_extsize = 0;
+	if (ip->i_d.di_version == 3 &&
+	    (ip->i_d.di_flags2 & XFS_DIFLAG2_COWEXTSIZE))
+		ip->i_d.di_cowextsize = fa->fsx_cowextsize >>
+				mp->m_sb.sb_blocklog;
+	else
+		ip->i_d.di_cowextsize = 0;
 
 	code = xfs_trans_commit(tp);
 
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 765849e..d907eb9 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -71,7 +71,7 @@
 	iomap->bdev = xfs_find_bdev_for_inode(VFS_I(ip));
 }
 
-static xfs_extlen_t
+xfs_extlen_t
 xfs_eof_alignment(
 	struct xfs_inode	*ip,
 	xfs_extlen_t		extsize)
diff --git a/fs/xfs/xfs_iomap.h b/fs/xfs/xfs_iomap.h
index a16b956..6d45cf0 100644
--- a/fs/xfs/xfs_iomap.h
+++ b/fs/xfs/xfs_iomap.h
@@ -31,6 +31,7 @@
 
 void xfs_bmbt_to_iomap(struct xfs_inode *, struct iomap *,
 		struct xfs_bmbt_irec *);
+xfs_extlen_t xfs_eof_alignment(struct xfs_inode *ip, xfs_extlen_t extsize);
 
 extern struct iomap_ops xfs_iomap_ops;
 extern struct iomap_ops xfs_xattr_iomap_ops;
diff --git a/fs/xfs/xfs_itable.c b/fs/xfs/xfs_itable.c
index ce73eb3..66e8817 100644
--- a/fs/xfs/xfs_itable.c
+++ b/fs/xfs/xfs_itable.c
@@ -66,7 +66,7 @@
 	if (!buffer || xfs_internal_inum(mp, ino))
 		return -EINVAL;
 
-	buf = kmem_alloc(sizeof(*buf), KM_SLEEP | KM_MAYFAIL);
+	buf = kmem_zalloc(sizeof(*buf), KM_SLEEP | KM_MAYFAIL);
 	if (!buf)
 		return -ENOMEM;
 
@@ -111,6 +111,12 @@
 	buf->bs_aextents = dic->di_anextents;
 	buf->bs_forkoff = XFS_IFORK_BOFF(ip);
 
+	if (dic->di_version == 3) {
+		if (dic->di_flags2 & XFS_DIFLAG2_COWEXTSIZE)
+			buf->bs_cowextsize = dic->di_cowextsize <<
+					mp->m_sb.sb_blocklog;
+	}
+
 	switch (dic->di_format) {
 	case XFS_DINODE_FMT_DEV:
 		buf->bs_rdev = ip->i_df.if_u2.if_rdev;
diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index d4707e6..8e89411 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -238,6 +238,7 @@
 	int			nimaps, eof = 0, error = 0;
 	bool			shared = false, trimmed = false;
 	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,
@@ -277,6 +278,10 @@
 	if (error)
 		goto out_unlock;
 
+	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,
@@ -927,18 +932,19 @@
 }
 
 /*
- * Update destination inode size, if necessary.
+ * Update destination inode size & cowextsize hint, if necessary.
  */
 STATIC int
 xfs_reflink_update_dest(
 	struct xfs_inode	*dest,
-	xfs_off_t		newlen)
+	xfs_off_t		newlen,
+	xfs_extlen_t		cowextsize)
 {
 	struct xfs_mount	*mp = dest->i_mount;
 	struct xfs_trans	*tp;
 	int			error;
 
-	if (newlen <= i_size_read(VFS_I(dest)))
+	if (newlen <= i_size_read(VFS_I(dest)) && cowextsize == 0)
 		return 0;
 
 	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_ichange, 0, 0, 0, &tp);
@@ -948,9 +954,17 @@
 	xfs_ilock(dest, XFS_ILOCK_EXCL);
 	xfs_trans_ijoin(tp, dest, XFS_ILOCK_EXCL);
 
-	trace_xfs_reflink_update_inode_size(dest, newlen);
-	i_size_write(VFS_I(dest), newlen);
-	dest->i_d.di_size = newlen;
+	if (newlen > i_size_read(VFS_I(dest))) {
+		trace_xfs_reflink_update_inode_size(dest, newlen);
+		i_size_write(VFS_I(dest), newlen);
+		dest->i_d.di_size = newlen;
+	}
+
+	if (cowextsize) {
+		dest->i_d.di_cowextsize = cowextsize;
+		dest->i_d.di_flags2 |= XFS_DIFLAG2_COWEXTSIZE;
+	}
+
 	xfs_trans_log_inode(tp, dest, XFS_ILOG_CORE);
 
 	error = xfs_trans_commit(tp);
@@ -1270,6 +1284,7 @@
 	xfs_fileoff_t		sfsbno, dfsbno;
 	xfs_filblks_t		fsblen;
 	int			error;
+	xfs_extlen_t		cowextsize;
 	bool			is_same;
 
 	if (!xfs_sb_version_hasreflink(&mp->m_sb))
@@ -1330,7 +1345,19 @@
 	if (error)
 		goto out_error;
 
-	error = xfs_reflink_update_dest(dest, destoff + len);
+	/*
+	 * Carry the cowextsize hint from src to dest if we're sharing the
+	 * entire source file to the entire destination file, the source file
+	 * has a cowextsize hint, and the destination file does not.
+	 */
+	cowextsize = 0;
+	if (srcoff == 0 && len == i_size_read(VFS_I(src)) &&
+	    (src->i_d.di_flags2 & XFS_DIFLAG2_COWEXTSIZE) &&
+	    destoff == 0 && len >= i_size_read(VFS_I(dest)) &&
+	    !(dest->i_d.di_flags2 & XFS_DIFLAG2_COWEXTSIZE))
+		cowextsize = src->i_d.di_cowextsize;
+
+	error = xfs_reflink_update_dest(dest, destoff + len, cowextsize);
 	if (error)
 		goto out_error;