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

[Goffredo Baroncelli <kreijack@inwind.it>]

On 11/01/2022 06.27, Anand Jain wrote:
> 
>>> 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.

Ok, I think that this case (where devt!=0 and bdev==NULL) should be inserted as comment in the structure before the devt field.

> 
> Thanks, Anand


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