diff --git a/fs/dax.c b/fs/dax.c
index 9fbbdaa784b4..21b47402b3dc 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1262,26 +1262,46 @@ static s64 dax_unshare_iter(struct iomap_iter *iter)
 {
 	struct iomap *iomap = &iter->iomap;
 	const struct iomap *srcmap = iomap_iter_srcmap(iter);
-	loff_t pos = iter->pos;
-	loff_t length = iomap_length(iter);
+	loff_t copy_pos = iter->pos;
+	u64 copy_len = iomap_length(iter);
+	u32 mod;
 	int id = 0;
 	s64 ret = 0;
 	void *daddr = NULL, *saddr = NULL;
 
 	if (!iomap_want_unshare_iter(iter))
-		return length;
+		return iomap_length(iter);
+
+	/*
+	 * Extend the file range to be aligned to fsblock/pagesize, because
+	 * we need to copy entire blocks, not just the byte range specified.
+	 * Invalidate the mapping because we're about to CoW.
+	 */
+	mod = offset_in_page(copy_pos);
+	if (mod) {
+		copy_len += mod;
+		copy_pos -= mod;
+	}
+
+	mod = offset_in_page(copy_pos + copy_len);
+	if (mod)
+		copy_len += PAGE_SIZE - mod;
+
+	invalidate_inode_pages2_range(iter->inode->i_mapping,
+				      copy_pos >> PAGE_SHIFT,
+				      (copy_pos + copy_len - 1) >> PAGE_SHIFT);
 
 	id = dax_read_lock();
-	ret = dax_iomap_direct_access(iomap, pos, length, &daddr, NULL);
+	ret = dax_iomap_direct_access(iomap, copy_pos, copy_len, &daddr, NULL);
 	if (ret < 0)
 		goto out_unlock;
 
-	ret = dax_iomap_direct_access(srcmap, pos, length, &saddr, NULL);
+	ret = dax_iomap_direct_access(srcmap, copy_pos, copy_len, &saddr, NULL);
 	if (ret < 0)
 		goto out_unlock;
 
-	if (copy_mc_to_kernel(daddr, saddr, length) == 0)
-		ret = length;
+	if (copy_mc_to_kernel(daddr, saddr, copy_len) == 0)
+		ret = iomap_length(iter);
 	else
 		ret = -EIO;