btrfs-progs: two staged filesystem creation

The filesystem existence on a device is manifested by the signature,
during the mkfs process we write it first and then create other
structures. Such filesystem is not valid and should not be registered
during device scan nor listed among devices from blkid.

This patch will introduce two staged creation. In the first phase, the
signature is wrong, but recognized as a partially created filesystem (by
open or scan helpers). Once we successfully create and write everything,
we fixup the signature. At this point automated scanning should find
a valid filesystem on all devices.

We can also rely on the partially created filesystem to do better error
handling during creation. We can just bail out and do not need to clean
up.

The partial signature is '!BHRfS_M', can be shown by

  btrfs inspect-internal dump-super -F image

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs-convert.c

@@ -2345,7 +2345,7 @@ static int do_convert(const char *devname, int datacsum, int packing,
 	}
 
 	root = open_ctree_fd(fd, devname, mkfs_cfg.super_bytenr,
-			     OPEN_CTREE_WRITES);
+			     OPEN_CTREE_WRITES | OPEN_CTREE_FS_PARTIAL);
 	if (!root) {
 		fprintf(stderr, "unable to open ctree\n");
 		goto fail;
@@ -2431,11 +2431,14 @@ static int do_convert(const char *devname, int datacsum, int packing,
 	}
 	is_btrfs = 1;
 
-	root = open_ctree_fd(fd, devname, 0, OPEN_CTREE_WRITES);
+	root = open_ctree_fd(fd, devname, 0,
+			OPEN_CTREE_WRITES | OPEN_CTREE_FS_PARTIAL);
 	if (!root) {
 		fprintf(stderr, "unable to open ctree\n");
 		goto fail;
 	}
+	root->fs_info->finalize_on_close = 1;
+	close_ctree(root);
 	close(fd);
 
 	printf("conversion complete.\n");

ctree.h

@@ -1032,6 +1032,7 @@ struct btrfs_fs_info {
 	unsigned int ignore_chunk_tree_error:1;
 	unsigned int avoid_meta_chunk_alloc:1;
 	unsigned int avoid_sys_chunk_alloc:1;
+	unsigned int finalize_on_close:1;
 
 	int (*free_extent_hook)(struct btrfs_trans_handle *trans,
 				struct btrfs_root *root,

disk-io.c

@@ -1225,6 +1225,7 @@ static struct btrfs_fs_info *__open_ctree_fd(int fp, const char *path,
 	struct extent_buffer *eb;
 	int ret;
 	int oflags;
+	unsigned sbflags = SBREAD_DEFAULT;
 
 	if (sb_bytenr == 0)
 		sb_bytenr = BTRFS_SUPER_INFO_OFFSET;
@@ -1247,9 +1248,18 @@ static struct btrfs_fs_info *__open_ctree_fd(int fp, const char *path,
 	if (flags & OPEN_CTREE_IGNORE_CHUNK_TREE_ERROR)
 		fs_info->ignore_chunk_tree_error = 1;
 
-	ret = btrfs_scan_fs_devices(fp, path, &fs_devices, sb_bytenr,
-		(flags & OPEN_CTREE_RECOVER_SUPER) ? SBREAD_RECOVER : SBREAD_DEFAULT,
-		(flags & OPEN_CTREE_NO_DEVICES));
+	if ((flags & OPEN_CTREE_RECOVER_SUPER)
+	     && (flags & OPEN_CTREE_FS_PARTIAL)) {
+		fprintf(stderr,
+		    "cannot open a partially created filesystem for recovery");
+		goto out;
+	}
+
+	if (flags & OPEN_CTREE_FS_PARTIAL)
+		sbflags = SBREAD_PARTIAL;
+
+	ret = btrfs_scan_fs_devices(fp, path, &fs_devices, sb_bytenr, sbflags,
+			(flags & OPEN_CTREE_NO_DEVICES));
 	if (ret)
 		goto out;
 
@@ -1272,7 +1282,7 @@ static struct btrfs_fs_info *__open_ctree_fd(int fp, const char *path,
 				sb_bytenr, SBREAD_RECOVER);
 	else
 		ret = btrfs_read_dev_super(fp, disk_super, sb_bytenr,
-				SBREAD_DEFAULT);
+				sbflags);
 	if (ret) {
 		printk("No valid btrfs found\n");
 		goto out_devices;
@@ -1401,8 +1411,12 @@ static int check_super(struct btrfs_super_block *sb, unsigned sbflags)
 	int csum_size;
 
 	if (btrfs_super_magic(sb) != BTRFS_MAGIC) {
-		error("superblock magic doesn't match");
-		return -EIO;
+		if (btrfs_super_magic(sb) == BTRFS_MAGIC_PARTIAL) {
+			if (!(sbflags & SBREAD_PARTIAL)) {
+				error("superblock magic doesn't match");
+				return -EIO;
+			}
+		}
 	}
 
 	csum_type = btrfs_super_csum_type(sb);
@@ -1747,6 +1761,15 @@ int close_ctree_fs_info(struct btrfs_fs_info *fs_info)
 		write_ctree_super(trans, root);
 		btrfs_free_transaction(root, trans);
 	}
+
+	if (fs_info->finalize_on_close) {
+		btrfs_set_super_magic(fs_info->super_copy, BTRFS_MAGIC);
+		root->fs_info->finalize_on_close = 0;
+		ret = write_all_supers(root);
+		if (ret)
+			fprintf(stderr,
+				"failed to write new super block err %d\n", ret);
+	}
 	btrfs_free_block_groups(fs_info);
 
 	free_fs_roots_tree(&fs_info->fs_root_tree);

disk-io.h

@@ -61,7 +61,10 @@ enum btrfs_open_ctree_flags {
 	 * It's useful for chunk corruption case.
 	 * Makes no sense for open_ctree variants returning btrfs_root.
 	 */
-	OPEN_CTREE_IGNORE_CHUNK_TREE_ERROR = (1 << 11)
+	OPEN_CTREE_IGNORE_CHUNK_TREE_ERROR = (1 << 11),
+
+	/* Allow to open a partially created filesystem */
+	OPEN_CTREE_FS_PARTIAL = (1 << 12),
 };
 
 /*
@@ -71,6 +74,12 @@ enum btrfs_read_sb_flags {
 	SBREAD_DEFAULT		= 0,
 	/* Reading superblock during recovery */
 	SBREAD_RECOVER		= (1 << 0),
+
+	/*
+	 * Read superblock with the fake signature, cannot be used with
+	 * SBREAD_RECOVER
+	 */
+	SBREAD_PARTIAL		= (1 << 1),
 };
 
 static inline u64 btrfs_sb_offset(int mirror)

mkfs.c

@@ -1708,7 +1708,7 @@ int main(int argc, char **argv)
 		exit(1);
 	}
 
-	root = open_ctree(file, 0, OPEN_CTREE_WRITES);
+	root = open_ctree(file, 0, OPEN_CTREE_WRITES | OPEN_CTREE_FS_PARTIAL);
 	if (!root) {
 		error("open ctree failed");
 		close(fd);
@@ -1858,6 +1858,11 @@ raid_groups:
 		list_all_devices(root);
 	}
 
+	/*
+	 * The filesystem is now fully set up, commit the remaining changes and
+	 * fix the signature as the last step before closing the devices.
+	 */
+	root->fs_info->finalize_on_close = 1;
 out:
 	ret = close_ctree(root);
 	BUG_ON(ret);

utils.c

@@ -238,6 +238,9 @@ static inline int write_temp_super(int fd, struct btrfs_super_block *sb,
  *
  * For now sys chunk array will be empty and dev_item is empty too.
  * They will be re-initialized at temp chunk tree setup.
+ *
+ * The superblock signature is not valid, denotes a partially created
+ * filesystem, needs to be finalized.
  */
 static int setup_temp_super(int fd, struct btrfs_mkfs_config *cfg,
 			    u64 root_bytenr, u64 chunk_bytenr)
@@ -276,7 +279,7 @@ static int setup_temp_super(int fd, struct btrfs_mkfs_config *cfg,
 
 	btrfs_set_super_bytenr(super, cfg->super_bytenr);
 	btrfs_set_super_num_devices(super, 1);
-	btrfs_set_super_magic(super, BTRFS_MAGIC);
+	btrfs_set_super_magic(super, BTRFS_MAGIC_PARTIAL);
 	btrfs_set_super_generation(super, 1);
 	btrfs_set_super_root(super, root_bytenr);
 	btrfs_set_super_chunk_root(super, chunk_bytenr);
@@ -1004,6 +1007,9 @@ out:
 
 /*
  * @fs_uuid - if NULL, generates a UUID, returns back the new filesystem UUID
+ *
+ * The superblock signature is not valid, denotes a partially created
+ * filesystem, needs to be finalized.
  */
 int make_btrfs(int fd, struct btrfs_mkfs_config *cfg,
 		struct btrfs_convert_context *cctx)
@@ -1064,7 +1070,7 @@ int make_btrfs(int fd, struct btrfs_mkfs_config *cfg,
 
 	btrfs_set_super_bytenr(&super, cfg->blocks[0]);
 	btrfs_set_super_num_devices(&super, 1);
-	btrfs_set_super_magic(&super, BTRFS_MAGIC);
+	btrfs_set_super_magic(&super, BTRFS_MAGIC_PARTIAL);
 	btrfs_set_super_generation(&super, 1);
 	btrfs_set_super_root(&super, cfg->blocks[1]);
 	btrfs_set_super_chunk_root(&super, cfg->blocks[3]);