diff --git a/source/ftl/map.c b/source/ftl/map.c
index a789912..3082945 100644
--- a/source/ftl/map.c
+++ b/source/ftl/map.c
@@ -85,13 +85,13 @@ int read_page_group(struct ftl_map *map,
 int read_page_desc(struct ftl_map *map,
 	struct ftl_page_desc *page_desc, uint32_t upage)
 {
-	uint32_t group_no, page, addr, offset;
+	uint32_t addr, offset;
 
-	group_no = upage >> map->log2_pages_per_group;
-	page = ((group_no + 1) << map->log2_pages_per_group) - 1;
-	addr = page << map->log2_page_size;
 	offset = sizeof(struct ftl_page_group) +
-		(upage & ((1 << map->log2_pages_per_group) - 1)) * sizeof *page_desc;
+		BIT_MASK(upage, map->log2_pages_per_group) * sizeof *page_desc;
+	upage = align(upage, map->log2_pages_per_group) +
+		(1 << map->log2_pages_per_group) - 1;
+	addr = upage << map->log2_page_size;
 
 	if (flash_read(map->dev, addr + offset, page_desc, sizeof *page_desc) == 0)
 		return -1;
@@ -109,14 +109,17 @@ int write_page_desc(struct ftl_map *map,
 	struct ftl_page_desc *page_desc)
 {
 	struct ftl_page_group group;
-	uint32_t group_no, page, addr, offset, head;
+	uint32_t upage, addr, offset, head;
 
-	group_no = map->head >> map->log2_pages_per_group;
-	page = ((group_no + 1) << map->log2_pages_per_group) - 1;
-	addr = page << map->log2_page_size;
+	upage = map->head;
+	offset = sizeof(struct ftl_page_group) +
+		BIT_MASK(upage, map->log2_pages_per_group) * sizeof *page_desc;
+	upage = align(upage, map->log2_pages_per_group) +
+		(1 << map->log2_pages_per_group) - 1;
+	addr = upage << map->log2_page_size;
 
 	/* Write the page group header. */
-	if (flash_is_erased(map->dev, page, 1)) {
+	if (flash_is_erased(map->dev, upage, 1)) {
 		memcpy(&group.magic, "FTL", sizeof group.magic);
 		group.epoch = map->epoch;
 		group.tail = map->tail;
@@ -125,9 +128,6 @@ int write_page_desc(struct ftl_map *map,
 			return -1;
 	}
 
-	offset = sizeof group + (map->head & ((1 << map->log2_pages_per_group) - 1)) *
-		sizeof *page_desc;
-
 	memcpy(page_desc->magic, "page", sizeof page_desc->magic);
 
 	if (flash_write(map->dev, addr + offset, page_desc, sizeof *page_desc) == 0)