Re: [PATCH 6/7] btrfs: add helper btrfs_num_devices() to deduce num_devices

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

On 07/19/2018 07:53 PM, David Sterba wrote:
> On Mon, Jul 16, 2018 at 10:58:11PM +0800, Anand Jain wrote:
>> When the replace is running the fs_devices::num_devices also includes
>> the replace device, however in some operations like device delete and
>> balance it needs the actual num_devices without the repalce devices, so
>> now the function btrfs_num_devices() just provides that.
> 
> We can't run any two from device delete, device replace or balance at
> the same time.
> 
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>> v2: add comments. Thanks Nikolay.
>>
>>   fs/btrfs/volumes.c | 32 ++++++++++++++++++--------------
>>   1 file changed, 18 insertions(+), 14 deletions(-)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index 0f4c512aa6b4..1c0b56374992 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -1844,6 +1844,21 @@ void btrfs_assign_next_active_device(struct btrfs_fs_info *fs_info,
>>   		fs_info->fs_devices->latest_bdev = next_device->bdev;
>>   }
>>   
>> +/* Returns btrfs_fs_devices::num_devices excluding replace device if any */
>> +static inline u64 btrfs_num_devices(struct btrfs_fs_info *fs_info)
>> +{
>> +	u64 num_devices = fs_info->fs_devices->num_devices;
>> +
>> +	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
>> +	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
>> +		WARN_ON(num_devices < 1);
>> +		num_devices--;
>> +	}
>> +	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);



> This does not make sense, besides that > btrfs_dev_replace_is_ongoing is
> always going to be false here,

  No. There is a way how balance and replace could co-exists.
  (theoretically, I didn't experiment it yet)
  . Start balance and pause it
  . Now start the replace
  . power-fail
  . The open_ctree() first starts the balance so it must check
  for the replace device otherwise our num_devices calculation will
  be wrong. IMO its not a good idea to remove the replace check here.

  For now a consolidation as in this patch is better.

Thanks, Anand


> the locking would need to cover the whole
> range where we want the num_devices to remain unchanged by other
> operatons.
> 
>> +
>> +	return num_devices;
>> +}
>> +
>>   int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
>>   		u64 devid)
>>   {
>> @@ -1857,13 +1872,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
>>   
>>   	mutex_lock(&uuid_mutex);
>>   
>> -	num_devices = fs_devices->num_devices;
>> -	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
>> -	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
>> -		WARN_ON(num_devices < 1);
>> -		num_devices--;
>> -	}
>> -	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
>> +	num_devices = btrfs_num_devices(fs_info);
>>   
>>   	ret = btrfs_check_raid_min_devices(fs_info, num_devices - 1);
>>   	if (ret)
>> @@ -3723,13 +3732,8 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
>>   		}
>>   	}
>>   
>> -	num_devices = fs_info->fs_devices->num_devices;
>> -	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
>> -	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
>> -		WARN_ON(num_devices < 1);
>> -		num_devices--;
>> -	}
>> -	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
>> +	num_devices = btrfs_num_devices(fs_info);
>> +
>>   	allowed = BTRFS_AVAIL_ALLOC_BIT_SINGLE | BTRFS_BLOCK_GROUP_DUP;
>>   	if (num_devices > 1)
>>   		allowed |= (BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1);
>> -- 
>> 2.7.0
>>
>> --
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