[PATCH 3/6] btrfs: Add handling for disk split-brain scenario during fsid change

[Nikolay Borisov <nborisov@suse.com>]

Even though FSID change without rewrite is a very quick operations it's
still possible to experience a split brain scenario if power loss
occurs at the right time. This patch handle the case where power
failure occurs while the first transaction (the one setting
FSID_CHANGING_V2) flag is being persisted on disk. This can cause
the btrfs_fs_device of this filesystem to be created by a device which:

 a) has the FSID_CHANGING_V2 flag set but its fsid value is intact

 b) or a device which doesn't have FSID_CHANGING_V2 flag set and its
 fsid value is intact

This situatian is trivially handled by the current find_fsid code since
in both cases the devices are going to be tread like ordinary devices.
Since btrfs is mounted always using the superblock of the latest
device (the one with higher generation number), meaning it will have
the FSID_CHANGING_V2 flag set, ensure it's being cleared. On the first
transaction commit following the mount all disks will have it cleared.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/disk-io.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index be2caf513e2f..9c2f46f8421a 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2831,10 +2831,10 @@ int open_ctree(struct super_block *sb,
 	 * the whole block of INFO_SIZE
 	 */
 	memcpy(fs_info->super_copy, bh->b_data, sizeof(*fs_info->super_copy));
-	memcpy(fs_info->super_for_commit, fs_info->super_copy,
-	       sizeof(*fs_info->super_for_commit));
 	brelse(bh);
 
+	disk_super = fs_info->super_copy;
+
 	ASSERT(!memcmp(fs_info->fs_devices->fsid, fs_info->super_copy->fsid,
 		       BTRFS_FSID_SIZE));
 
@@ -2844,6 +2844,15 @@ int open_ctree(struct super_block *sb,
 				BTRFS_FSID_SIZE));
 	}
 
+	features = btrfs_super_flags(disk_super);
+	if (features & BTRFS_SUPER_FLAG_CHANGING_FSID_v2) {
+		features &= ~BTRFS_SUPER_FLAG_CHANGING_FSID_v2;
+		btrfs_set_super_flags(disk_super, features);
+		btrfs_info(fs_info, "Found metadata uuid in progress flag. Clearing\n");
+	}
+
+	memcpy(fs_info->super_for_commit, fs_info->super_copy,
+	       sizeof(*fs_info->super_for_commit));
 
 	ret = btrfs_validate_mount_super(fs_info);
 	if (ret) {
@@ -2852,7 +2861,6 @@ int open_ctree(struct super_block *sb,
 		goto fail_alloc;
 	}
 
-	disk_super = fs_info->super_copy;
 	if (!btrfs_super_root(disk_super))
 		goto fail_alloc;
 
-- 
2.7.4