Re: [PATCH v4 3/4] btrfs: add device major-minor info in the struct btrfs_device

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

>> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
>> index 76215de8ce34..ebfe5dc73e26 100644
>> --- a/fs/btrfs/volumes.h
>> +++ b/fs/btrfs/volumes.h
>> @@ -73,6 +73,8 @@ struct btrfs_device {
>>       /* the mode sent to blkdev_get */
>>       fmode_t mode;
>> +    /* Device's major-minor number */
>> +    dev_t devt;
> 
> What is the relation between
> 
>      device->devt
> 
> and
> 
>      device->bdev->bd_dev
> 
> ?

  Both are same. device->bdev gets an update at the time device open. 
Otherwise, it is null.

> I assumed that there are situation where there is no a device connected 
> to a btrfs_device
> structure (e.g. a degraded mounted filesystem where a device is 
> missing); in this case
> does devt == 0 ?

  Even for the missing device we do call add_missing_dev() -> 
btrfs_alloc_device() that will ensure devt == 0.

> But are there cases where devt != 0 (a device is associated to a block 
> device structure) and bdev == NULL ?

  It is possible- When we unmount, the btrfs_device continues to exist 
and, both device->name and device->devt shall not be NULL/0; However, 
device->bdev will be NULL; If the device is scanned again with a 
different uuid, then the free_stale_device() is called to check and free 
the old/stale struct btrfs_device.

Thanks, Anand