diff --git a/midx.c b/midx.c
index 2af4452165..f7c3a54a33 100644
--- a/midx.c
+++ b/midx.c
@@ -461,6 +461,9 @@ struct write_midx_context {
 
 	struct pack_midx_entry *entries;
 	uint32_t entries_nr;
+
+	uint32_t *pack_perm;
+	unsigned large_offsets_needed:1;
 };
 
 static void add_pack_to_midx(const char *full_path, size_t full_path_len,
@@ -736,27 +739,27 @@ static size_t write_midx_oid_lookup(struct hashfile *f,
 	return written;
 }
 
-static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_needed,
-					uint32_t *perm,
-					struct pack_midx_entry *objects, uint32_t nr_objects)
+static size_t write_midx_object_offsets(struct hashfile *f,
+					void *data)
 {
-	struct pack_midx_entry *list = objects;
+	struct write_midx_context *ctx = (struct write_midx_context *)data;
+	struct pack_midx_entry *list = ctx->entries;
 	uint32_t i, nr_large_offset = 0;
 	size_t written = 0;
 
-	for (i = 0; i < nr_objects; i++) {
+	for (i = 0; i < ctx->entries_nr; i++) {
 		struct pack_midx_entry *obj = list++;
 
-		if (perm[obj->pack_int_id] == PACK_EXPIRED)
+		if (ctx->pack_perm[obj->pack_int_id] == PACK_EXPIRED)
 			BUG("object %s is in an expired pack with int-id %d",
 			    oid_to_hex(&obj->oid),
 			    obj->pack_int_id);
 
-		hashwrite_be32(f, perm[obj->pack_int_id]);
+		hashwrite_be32(f, ctx->pack_perm[obj->pack_int_id]);
 
-		if (large_offset_needed && obj->offset >> 31)
+		if (ctx->large_offsets_needed && obj->offset >> 31)
 			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
-		else if (!large_offset_needed && obj->offset >> 32)
+		else if (!ctx->large_offsets_needed && obj->offset >> 32)
 			BUG("object %s requires a large offset (%"PRIx64") but the MIDX is not writing large offsets!",
 			    oid_to_hex(&obj->oid),
 			    obj->offset);
@@ -805,13 +808,11 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct write_midx_context ctx = { 0 };
-	uint32_t *pack_perm = NULL;
 	uint64_t written = 0;
 	uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
 	uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
 	uint32_t num_large_offsets = 0;
 	struct progress *progress = NULL;
-	int large_offsets_needed = 0;
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = 0;
@@ -857,11 +858,12 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 
 	ctx.entries = get_sorted_entries(ctx.m, ctx.info, ctx.nr, &ctx.entries_nr);
 
+	ctx.large_offsets_needed = 0;
 	for (i = 0; i < ctx.entries_nr; i++) {
 		if (ctx.entries[i].offset > 0x7fffffff)
 			num_large_offsets++;
 		if (ctx.entries[i].offset > 0xffffffff)
-			large_offsets_needed = 1;
+			ctx.large_offsets_needed = 1;
 	}
 
 	QSORT(ctx.info, ctx.nr, pack_info_compare);
@@ -900,13 +902,13 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	 *
 	 * pack_perm[old_id] = new_id
 	 */
-	ALLOC_ARRAY(pack_perm, ctx.nr);
+	ALLOC_ARRAY(ctx.pack_perm, ctx.nr);
 	for (i = 0; i < ctx.nr; i++) {
 		if (ctx.info[i].expired) {
 			dropped_packs++;
-			pack_perm[ctx.info[i].orig_pack_int_id] = PACK_EXPIRED;
+			ctx.pack_perm[ctx.info[i].orig_pack_int_id] = PACK_EXPIRED;
 		} else {
-			pack_perm[ctx.info[i].orig_pack_int_id] = i - dropped_packs;
+			ctx.pack_perm[ctx.info[i].orig_pack_int_id] = i - dropped_packs;
 		}
 	}
 
@@ -927,7 +929,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 		close_midx(ctx.m);
 
 	cur_chunk = 0;
-	num_chunks = large_offsets_needed ? 5 : 4;
+	num_chunks = ctx.large_offsets_needed ? 5 : 4;
 
 	if (ctx.nr - dropped_packs == 0) {
 		error(_("no pack files to index."));
@@ -954,7 +956,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 
 	cur_chunk++;
 	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + ctx.entries_nr * MIDX_CHUNK_OFFSET_WIDTH;
-	if (large_offsets_needed) {
+	if (ctx.large_offsets_needed) {
 		chunk_ids[cur_chunk] = MIDX_CHUNKID_LARGEOFFSETS;
 
 		cur_chunk++;
@@ -1004,7 +1006,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 				break;
 
 			case MIDX_CHUNKID_OBJECTOFFSETS:
-				written += write_midx_object_offsets(f, large_offsets_needed, pack_perm, ctx.entries, ctx.entries_nr);
+				written += write_midx_object_offsets(f, &ctx);
 				break;
 
 			case MIDX_CHUNKID_LARGEOFFSETS:
@@ -1039,7 +1041,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 
 	free(ctx.info);
 	free(ctx.entries);
-	free(pack_perm);
+	free(ctx.pack_perm);
 	free(midx_name);
 	return result;
 }
