diff --git a/fs/ntfs/runlist.c b/fs/ntfs/runlist.c
index f13d28c15887..76ba6ab72b8a 100644
--- a/fs/ntfs/runlist.c
+++ b/fs/ntfs/runlist.c
@@ -47,7 +47,7 @@ static inline void ntfs_rl_mm(struct runlist_element *base, int dst, int src, in
  * @srcbase.
  */
 static inline void ntfs_rl_mc(struct runlist_element *dstbase, int dst,
-		struct runlist_element *srcbase, int src, int size)
+		const struct runlist_element *srcbase, int src, int size)
 {
 	if (likely(size > 0))
 		memcpy(dstbase + dst, srcbase + src, size * sizeof(*dstbase));
@@ -152,8 +152,8 @@ static inline struct runlist_element *ntfs_rl_realloc_nofail(struct runlist_elem
  * Return: true   Success, the runlists can be merged.
  *	   false  Failure, the runlists cannot be merged.
  */
-static inline bool ntfs_are_rl_mergeable(struct runlist_element *dst,
-		struct runlist_element *src)
+static inline bool ntfs_are_rl_mergeable(const struct runlist_element *dst,
+		const struct runlist_element *src)
 {
 	/* We can merge unmapped regions even if they are misaligned. */
 	if ((dst->lcn == LCN_RL_NOT_MAPPED) && (src->lcn == LCN_RL_NOT_MAPPED))
@@ -186,7 +186,8 @@ static inline bool ntfs_are_rl_mergeable(struct runlist_element *dst,
  *
  * It is up to the caller to serialize access to the runlists @dst and @src.
  */
-static inline void __ntfs_rl_merge(struct runlist_element *dst, struct runlist_element *src)
+static inline void __ntfs_rl_merge(struct runlist_element *dst,
+		const struct runlist_element *src)
 {
 	dst->length += src->length;
 }
@@ -214,10 +215,11 @@ static inline void __ntfs_rl_merge(struct runlist_element *dst, struct runlist_e
  * On error, return -errno. Both runlists are left unmodified.
  */
 static inline struct runlist_element *ntfs_rl_append(struct runlist_element *dst,
-		int dsize, struct runlist_element *src, int ssize, int loc,
+		int dsize, const struct runlist_element *src, int ssize, int loc,
 		size_t *new_size)
 {
 	bool right = false;	/* Right end of @src needs merging. */
+	s64 right_length = 0;	/* Length of the run merged at the right. */
 	int marker;		/* End of the inserted runs. */
 
 	/* First, check if the right hand end needs merging. */
@@ -235,9 +237,9 @@ static inline struct runlist_element *ntfs_rl_append(struct runlist_element *dst
 	 * original runlists.
 	 */
 
-	/* First, merge the right hand end, if necessary. */
+	/* Remember the run that the right hand end absorbs, if any. */
 	if (right)
-		__ntfs_rl_merge(src + ssize - 1, dst + loc + 1);
+		right_length = dst[loc + 1].length;
 
 	/* First run after the @src runs that have been inserted. */
 	marker = loc + ssize + 1;
@@ -246,6 +248,9 @@ static inline struct runlist_element *ntfs_rl_append(struct runlist_element *dst
 	ntfs_rl_mm(dst, marker, loc + 1 + right, dsize - (loc + 1 + right));
 	ntfs_rl_mc(dst, loc + 1, src, 0, ssize);
 
+	/* Merge the right hand end into the copy, leaving @src intact. */
+	dst[marker - 1].length += right_length;
+
 	/* Adjust the size of the preceding hole. */
 	dst[loc].length = dst[loc + 1].vcn - dst[loc].vcn;
 
@@ -279,7 +284,7 @@ static inline struct runlist_element *ntfs_rl_append(struct runlist_element *dst
  * On error, return -errno. Both runlists are left unmodified.
  */
 static inline struct runlist_element *ntfs_rl_insert(struct runlist_element *dst,
-		int dsize, struct runlist_element *src, int ssize, int loc,
+		int dsize, const struct runlist_element *src, int ssize, int loc,
 		size_t *new_size)
 {
 	bool left = false;	/* Left end of @src needs merging. */
@@ -374,12 +379,13 @@ static inline struct runlist_element *ntfs_rl_insert(struct runlist_element *dst
  * On error, return -errno. Both runlists are left unmodified.
  */
 static inline struct runlist_element *ntfs_rl_replace(struct runlist_element *dst,
-		int dsize, struct runlist_element *src, int ssize, int loc,
+		int dsize, const struct runlist_element *src, int ssize, int loc,
 		size_t *new_size)
 {
 	int delta;
 	bool left = false;	/* Left end of @src needs merging. */
 	bool right = false;	/* Right end of @src needs merging. */
+	s64 right_length = 0;	/* Length of the run merged at the right. */
 	int tail;		/* Start of tail of @dst. */
 	int marker;		/* End of the inserted runs. */
 
@@ -405,9 +411,12 @@ static inline struct runlist_element *ntfs_rl_replace(struct runlist_element *ds
 	 * original runlists.
 	 */
 
-	/* First, merge the left and right ends, if necessary. */
+	/*
+	 * Remember the run that the right hand end absorbs, if any, then
+	 * merge the left hand end.
+	 */
 	if (right)
-		__ntfs_rl_merge(src + ssize - 1, dst + loc + 1);
+		right_length = dst[loc + 1].length;
 	if (left)
 		__ntfs_rl_merge(dst + loc - 1, src);
 	/*
@@ -432,6 +441,13 @@ static inline struct runlist_element *ntfs_rl_replace(struct runlist_element *ds
 	ntfs_rl_mm(dst, marker, tail, dsize - tail);
 	ntfs_rl_mc(dst, loc, src, left, ssize - left);
 
+	/*
+	 * Merge the right hand end into the last run of @src, now in @dst at
+	 * @marker - 1 (or into @dst[@loc - 1] if that absorbed all of @src),
+	 * leaving @src intact.
+	 */
+	dst[marker - 1].length += right_length;
+
 	/* We may have changed the length of the file, so fix the end marker. */
 	if (dsize - tail > 0 && dst[marker].lcn == LCN_ENOENT)
 		dst[marker].vcn = dst[marker - 1].vcn + dst[marker - 1].length;
@@ -461,7 +477,7 @@ static inline struct runlist_element *ntfs_rl_replace(struct runlist_element *ds
  * On error, return -errno. Both runlists are left unmodified.
  */
 static inline struct runlist_element *ntfs_rl_split(struct runlist_element *dst, int dsize,
-		struct runlist_element *src, int ssize, int loc,
+		const struct runlist_element *src, int ssize, int loc,
 		size_t *new_size)
 {
 	/* Space required: @dst size + @src size + one new hole. */
@@ -488,11 +504,12 @@ static inline struct runlist_element *ntfs_rl_split(struct runlist_element *dst,
 }
 
 /*
- * ntfs_runlists_merge - merge two runlists into one
+ * __ntfs_runlists_merge - merge two runlists into one
  * @d_runlist: destination runlist structure to merge into
  * @srl: source runlist to merge from
  * @s_rl_count: number of elements in @srl (0 to auto-detect)
  * @new_rl_count: on success, set to the new combined runlist size
+ * @keep_srl: leave @srl to the caller instead of deallocating it
  *
  * First we sanity check the two runlists @srl and @drl to make sure that they
  * are sensible and can be merged. The runlist @srl must be either after the
@@ -515,13 +532,14 @@ static inline struct runlist_element *ntfs_rl_split(struct runlist_element *dst,
  * On success, return a pointer to the new, combined, runlist. Note, both
  * runlists @drl and @srl are deallocated before returning so you cannot use
  * the pointers for anything any more. (Strictly speaking the returned runlist
- * may be the same as @dst but this is irrelevant.)
+ * may be the same as @dst but this is irrelevant.)  If @keep_srl is true,
+ * @srl is neither modified nor deallocated, and @drl must not be NULL.
  *
  * On error, return -errno. Both runlists are left unmodified.
  */
-struct runlist_element *ntfs_runlists_merge(struct runlist *d_runlist,
-				     struct runlist_element *srl, size_t s_rl_count,
-				     size_t *new_rl_count)
+static struct runlist_element *__ntfs_runlists_merge(struct runlist *d_runlist,
+		struct runlist_element *srl, size_t s_rl_count,
+		size_t *new_rl_count, bool keep_srl)
 {
 	int di, si;		/* Current index into @[ds]rl. */
 	int sstart;		/* First index with lcn > LCN_RL_NOT_MAPPED. */
@@ -553,6 +571,8 @@ struct runlist_element *ntfs_runlists_merge(struct runlist *d_runlist,
 
 	/* Check for the case where the first mapping is being done now. */
 	if (unlikely(!drl)) {
+		if (WARN_ON(keep_srl))
+			return ERR_PTR(-EINVAL);
 		drl = srl;
 		/* Complete the source runlist if necessary. */
 		if (unlikely(drl[0].vcn)) {
@@ -582,7 +602,8 @@ struct runlist_element *ntfs_runlists_merge(struct runlist *d_runlist,
 		si++;
 
 	/* Can't have an entirely unmapped source runlist. */
-	WARN_ON(!srl[si].length);
+	if (WARN_ON(!srl[si].length))
+		return ERR_PTR(-EINVAL);
 
 	/* Record the starting points. */
 	sstart = si;
@@ -652,7 +673,8 @@ struct runlist_element *ntfs_runlists_merge(struct runlist *d_runlist,
 		ntfs_error(NULL, "Merge failed.");
 		return drl;
 	}
-	kvfree(srl);
+	if (!keep_srl)
+		kvfree(srl);
 	if (marker) {
 		ntfs_debug("Triggering marker code.");
 		for (ds = dend; drl[ds].length; ds++)
@@ -713,6 +735,44 @@ struct runlist_element *ntfs_runlists_merge(struct runlist *d_runlist,
 	return drl;
 }
 
+/*
+ * ntfs_runlists_merge - merge two runlists into one
+ * @d_runlist: destination runlist structure to merge into
+ * @srl: source runlist to merge from, deallocated on success
+ * @s_rl_count: number of elements in @srl (0 to auto-detect)
+ * @new_rl_count: on success, set to the new combined runlist size
+ *
+ * See __ntfs_runlists_merge().
+ */
+struct runlist_element *ntfs_runlists_merge(struct runlist *d_runlist,
+		struct runlist_element *srl, size_t s_rl_count,
+		size_t *new_rl_count)
+{
+	return __ntfs_runlists_merge(d_runlist, srl, s_rl_count, new_rl_count,
+			false);
+}
+
+/*
+ * ntfs_runlists_merge_keep_src - merge two runlists, keeping the source
+ * @d_runlist: destination runlist structure to merge into, not empty
+ * @srl: source runlist to merge from
+ * @s_rl_count: number of elements in @srl (0 to auto-detect)
+ * @new_rl_count: on success, set to the new combined runlist size
+ *
+ * Like ntfs_runlists_merge(), but @srl is neither modified nor deallocated,
+ * on success or on error, and remains the caller's to kvfree().  Even when
+ * runs of @srl are coalesced with runs of @d_runlist, @srl keeps describing
+ * exactly its own runs, e.g. the clusters a caller has to free to undo the
+ * merge.
+ */
+struct runlist_element *ntfs_runlists_merge_keep_src(struct runlist *d_runlist,
+		struct runlist_element *srl, size_t s_rl_count,
+		size_t *new_rl_count)
+{
+	return __ntfs_runlists_merge(d_runlist, srl, s_rl_count, new_rl_count,
+			true);
+}
+
 /*
  * ntfs_mapping_pairs_decompress - convert mapping pairs array to runlist
  * @vol: ntfs volume
diff --git a/fs/ntfs/runlist.h b/fs/ntfs/runlist.h
index cc92e09fbf68..0d4c32b1a338 100644
--- a/fs/ntfs/runlist.h
+++ b/fs/ntfs/runlist.h
@@ -68,6 +68,9 @@ enum {
 struct runlist_element *ntfs_runlists_merge(struct runlist *d_runlist,
 		struct runlist_element *srl, size_t s_rl_count,
 		size_t *new_rl_count);
+struct runlist_element *ntfs_runlists_merge_keep_src(struct runlist *d_runlist,
+		struct runlist_element *srl, size_t s_rl_count,
+		size_t *new_rl_count);
 struct runlist_element *ntfs_mapping_pairs_decompress(const struct ntfs_volume *vol,
 		const struct attr_record *attr, struct runlist *old_runlist,
 		size_t *new_rl_count);