btrfs: add members to fs_devices to track fsid changes

In order to gracefully handle split-brain scenario during fsid change
(which are very unlikely, yet possible), two more pieces of information
will be necessary:

1. The highest generation number among all devices registered to a
   particular btrfs_fs_devices

2. A boolean flag whether a given btrfs_fs_devices was created by a
   device which had the FSID_CHANGING_V2 flag set.

This is a preparatory patch and just introduces the variables as well
as code which sets them, their actual use is going to happen in a later
patch.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -792,6 +792,8 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 	u64 devid = btrfs_stack_device_id(&disk_super->dev_item);
 	bool has_metadata_uuid = (btrfs_super_incompat_flags(disk_super) &
 		BTRFS_FEATURE_INCOMPAT_METADATA_UUID);
+	bool fsid_change_in_progress = (btrfs_super_flags(disk_super) &
+					BTRFS_SUPER_FLAG_CHANGING_FSID_V2);
 
 	if (has_metadata_uuid)
 		fs_devices = find_fsid(disk_super->fsid, disk_super->metadata_uuid);
@@ -805,6 +807,8 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 		else
 			fs_devices = alloc_fs_devices(disk_super->fsid, NULL);
 
+		fs_devices->fsid_change = fsid_change_in_progress;
+
 		if (IS_ERR(fs_devices))
 			return ERR_CAST(fs_devices);
 
@@ -940,8 +944,11 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 	 * it back. We need it to pick the disk with largest generation
 	 * (as above).
 	 */
-	if (!fs_devices->opened)
+	if (!fs_devices->opened) {
 		device->generation = found_transid;
+		fs_devices->latest_generation = max_t(u64, found_transid,
+						fs_devices->latest_generation);
+	}
 
 	fs_devices->total_devices = btrfs_super_num_devices(disk_super);
 

fs/btrfs/volumes.h

@@ -211,6 +211,7 @@ BTRFS_DEVICE_GETSET_FUNCS(bytes_used);
 struct btrfs_fs_devices {
 	u8 fsid[BTRFS_FSID_SIZE]; /* FS specific uuid */
 	u8 metadata_uuid[BTRFS_FSID_SIZE];
+	bool fsid_change;
 	struct list_head fs_list;
 
 	u64 num_devices;
@@ -219,6 +220,10 @@ struct btrfs_fs_devices {
 	u64 missing_devices;
 	u64 total_rw_bytes;
 	u64 total_devices;
+
+	/* Highest generation number of seen devices */
+	u64 latest_generation;
+
 	struct block_device *latest_bdev;
 
 	/* all of the devices in the FS, protected by a mutex