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>

1 files changed, 8 insertions, 2 deletions

diff --git a/utils.c b/utils.c
index e7195b53..cec7c738 100644
--- a/utils.c
+++ b/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]);