ftl: fix bug in root recovery by marking page descriptors with a magic value

include/ftl.h

@@ -9,6 +9,7 @@ struct ftl_page_group {
 } __attribute__((packed));
 
 struct ftl_page_desc {
+	char magic[4];
 	uint32_t va;
 	uint32_t nused_pages;
 	uint32_t subtrees[32];

source/ftl/map.c

@@ -77,6 +77,9 @@ int read_page_desc(struct ftl_map *map,
 	if (flash_read(map->dev, addr + offset, page_desc, sizeof *page_desc) == 0)
 		return -1;
 
+	if (memcmp(page_desc->magic, "page", sizeof page_desc->magic) != 0)
+		return -1;
+
 	return 0;
 }
 
@@ -84,7 +87,7 @@ int read_page_desc(struct ftl_map *map,
  * increments the head to point to the next free user page.
  */
 int write_page_desc(struct ftl_map *map,
-	const struct ftl_page_desc *page_desc)
+	struct ftl_page_desc *page_desc)
 {
 	struct ftl_page_group group;
 	uint32_t group_no, page, addr, offset, head;
@@ -106,6 +109,8 @@ int write_page_desc(struct ftl_map *map,
 	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)
 		return -1;
 
@@ -126,7 +131,7 @@ int write_page_desc(struct ftl_map *map,
  * page.
  */
 int write_upage(struct ftl_map *map, const void *page,
-	const struct ftl_page_desc *page_desc)
+	struct ftl_page_desc *page_desc)
 {
 	if (prepare_head(map) < 0)
 		return -1;
@@ -265,14 +270,10 @@ static int find_root(struct ftl_map *map, uint32_t group)
 
 	upage = group << map->log2_pages_per_group;
 
-	do {
+	while (read_page_desc(map, &page_desc, upage) == 0) {
 		map->root = upage;
 		upage = next_upage(map, upage);
-
-		if (read_page_desc(map, &page_desc, upage) < 0)
-			return -1;
-	} while (page_desc.va != UINT32_MAX ||
-	         page_desc.nused_pages == 0);
+	}
 
 	return 0;
 }

source/ftl/map.h

@@ -7,8 +7,8 @@ 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);
 int write_page_desc(struct ftl_map *map,
-	const struct ftl_page_desc *page_desc);
+	struct ftl_page_desc *page_desc);
 int write_upage(struct ftl_map *map, const void *page,
-	const struct ftl_page_desc *page_desc);
+	struct ftl_page_desc *page_desc);
 int trace_path(struct ftl_map *map, struct ftl_page_desc *new_page_desc,
 	uint32_t *page, uint32_t va);