Re: [PATCH 3/7] btrfs: do device clone using the btrfs_scan_one_device

[Anand Jain <anand.jain@oracle.com>]

On 07/19/2018 08:31 PM, David Sterba wrote:
> On Mon, Jul 16, 2018 at 10:58:08PM +0800, Anand Jain wrote:
>> When we add a device to the RO mounted seed device, it becomes a
>> RW sprout FS. The following steps are used to hold the seed and
>> sprout fs_devices.
>>   (first two steps are not mandatory for the sprouting, they are there
>>    to ensure the seed device remains in the scanned state)
>>    . Clone the (mounted) fs_devices, lets call it as old_devices
>>    . Now add old_devices to fs_uuids (yeah, there is duplicate fsid in the
>>      list, as we are under uuid_mutex so its fine).
>>    . Alloc a new fs_devices, lets call it as seed_devices
>>    . Copy fs_devices into the seed_devices
>>    . Move fs_devices::devices into seed_devices::devices
>>    . Bring seed_devices to under fs_devices::seed
>>      (fs_devices->seed = seed_devices)
>>    . Assign a new FSID to the fs_devices and add the new writable device
>>      to the fs_devices.
> 
>> This patch makes the following changes..
>> As we clone fs_devices to make sure the device remains scanned after the
>> sprouting. So use the btrfs_scan_one_device() code instead. And do it
>> at the end of the sprouting.
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>>   fs/btrfs/volumes.c | 21 ++++++++++++---------
>>   1 file changed, 12 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index 8450bcfed4cb..c6f3f0dfbabe 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -2179,7 +2179,7 @@ int btrfs_find_device_by_devspec(struct btrfs_fs_info *fs_info, u64 devid,
>>   static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
>>   {
>>   	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
>> -	struct btrfs_fs_devices *old_devices;
>> +	struct btrfs_fs_devices *old_fs_devices;
>>   	struct btrfs_fs_devices *seed_devices;
>>   	struct btrfs_super_block *disk_super = fs_info->super_copy;
>>   	struct btrfs_device *device;
>> @@ -2193,14 +2193,6 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
>>   	if (IS_ERR(seed_devices))
>>   		return PTR_ERR(seed_devices);
>>   
>> -	old_devices = clone_fs_devices(fs_devices);
>> -	if (IS_ERR(old_devices)) {
>> -		kfree(seed_devices);
>> -		return PTR_ERR(old_devices);
>> -	}
>> -
>> -	list_add(&old_devices->fs_list, &fs_uuids);
>> -
>>   	memcpy(seed_devices, fs_devices, sizeof(*seed_devices));
>>   	seed_devices->opened = 1;
>>   	INIT_LIST_HEAD(&seed_devices->devices);
>> @@ -2233,6 +2225,17 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
>>   		      ~BTRFS_SUPER_FLAG_SEEDING;
>>   	btrfs_set_super_flags(disk_super, super_flags);
>>   
>> +	/*
>> +	 * As the above code hijacked the original seed fs_devices, now
>> +	 * create a new one for the original seed FSID.
>> +	 */
>> +	list_for_each_entry(device, &fs_devices->seed->devices, dev_list) {
>> +		if (!device->name)
>> +			continue;
>> +		btrfs_scan_one_device(device->name->str, FMODE_READ,
>> +				      fs_info->bdev_holder, &old_fs_devices);
> 
> So this is clone_fs_devices vs btrfs_scan_one_device approach. The clone
> only copies the devices from memory, while scan reads the superblock
> from the block device.

  Right.

> The changelog describes how the new fsdevices is built but does not
> explain why the new method is better than the current one. The end
> result is possibly the same and the current code does the necessary work
> to add the new fsdevices, while scanning will add the new fsid.
> 
> The rest of code adding the new device in device_list_add and inside the
> loop in clone_fs_devices is equivalent.

  Its just a code consolidate patch.

Thanks, Anand

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