diff --git a/Documentation/git-multi-pack-index.adoc b/Documentation/git-multi-pack-index.adoc
index 631d5c7..b6cd0d7 100644
--- a/Documentation/git-multi-pack-index.adoc
+++ b/Documentation/git-multi-pack-index.adoc
@@ -38,10 +38,13 @@
 +
 --
 	--preferred-pack=<pack>::
-		Optionally specify the tie-breaking pack used when
-		multiple packs contain the same object. `<pack>` must
-		contain at least one object. If not given, ties are
-		broken in favor of the pack with the lowest mtime.
+		When specified, break ties in favor of this pack when
+		there are additional copies of its objects in other
+		packs. Ties for objects not found in the preferred
+		pack are always resolved in favor of the copy in the
+		pack with the highest mtime. If unspecified, the pack
+		with the lowest mtime is used by default. The
+		preferred pack must have at least one object.
 
 	--[no-]bitmap::
 		Control whether or not a multi-pack bitmap is written.
diff --git a/git-compat-util.h b/git-compat-util.h
index 36b9577..4678e21 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -668,6 +668,22 @@ static inline int cast_size_t_to_int(size_t a)
 	return (int)a;
 }
 
+static inline uint64_t u64_mult(uint64_t a, uint64_t b)
+{
+	if (unsigned_mult_overflows(a, b))
+		die("uint64_t overflow: %"PRIuMAX" * %"PRIuMAX,
+		    (uintmax_t)a, (uintmax_t)b);
+	return a * b;
+}
+
+static inline uint64_t u64_add(uint64_t a, uint64_t b)
+{
+	if (unsigned_add_overflows(a, b))
+		die("uint64_t overflow: %"PRIuMAX" + %"PRIuMAX,
+		    (uintmax_t)a, (uintmax_t)b);
+	return a + b;
+}
+
 /*
  * Limit size of IO chunks, because huge chunks only cause pain.  OS X
  * 64-bit is buggy, returning EINVAL if len >= INT_MAX; and even in
diff --git a/midx-write.c b/midx-write.c
index dd3b307..ba4a949 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1566,7 +1566,7 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
 					  _("Counting referenced objects"),
 					  m->num_objects);
 	for (i = 0; i < m->num_objects; i++) {
-		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
 		count[pack_int_id]++;
 		display_progress(progress, i + 1);
 	}
@@ -1697,21 +1697,31 @@ static void fill_included_packs_batch(struct repository *r,
 
 	total_size = 0;
 	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
-		int pack_int_id = pack_info[i].pack_int_id;
+		uint32_t pack_int_id = pack_info[i].pack_int_id;
 		struct packed_git *p = m->packs[pack_int_id];
-		size_t expected_size;
+		uint64_t expected_size;
 
 		if (!want_included_pack(r, m, pack_kept_objects, pack_int_id))
 			continue;
 
-		expected_size = st_mult(p->pack_size,
-					pack_info[i].referenced_objects);
+		/*
+		 * Use shifted integer arithmetic to calculate the
+		 * expected pack size to ~4 significant digits without
+		 * overflow for packsizes less that 1PB.
+		 */
+		expected_size = (uint64_t)pack_info[i].referenced_objects << 14;
 		expected_size /= p->num_objects;
+		expected_size = u64_mult(expected_size, p->pack_size);
+		expected_size = u64_add(expected_size, 1u << 13) >> 14;
 
 		if (expected_size >= batch_size)
 			continue;
 
-		total_size += expected_size;
+		if (unsigned_add_overflows(total_size, (size_t)expected_size))
+			total_size = SIZE_MAX;
+		else
+			total_size += expected_size;
+
 		include_pack[pack_int_id] = 1;
 	}