Re: [PATCH 07/16] btrfs: add proper safety check before resuming dev-replace

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

On 04/07/2018 02:42 PM, Anand Jain wrote:
> 
> 
> On 04/04/2018 02:34 AM, David Sterba wrote:
>> The device replace is paused by unmount or read only remount, and
>> resumed on next mount or write remount.
>>
>> The exclusive status should be checked properly as it's a global
>> invariant and we must not allow 2 operations run. In this case, the
>> balance can be also paused and resumed under same conditions. It's
>> always checked first so dev-replace could see the EXCL_OP already taken,
>> BUT, the ioctl would never let start both at the same time.
>>
>> Replace the WARN_ON with message and return 0, indicating no error as
>> this is purely theoretical and the user will be informed. Resolving that
>> manually should be possible by waiting for the other operation to finish
>> or cancel the paused state.
> 
>   So if there is a paused balance and replace is triggered, a write
>   remount will reverse the process? 

  Ok, I am answering my own question:
  Even if the balance is paused that's considered as an exclusive
  operation in progress and does not let replace to start. So there
  is no scenario where paused-balance and replace could co-exist.

  So an asset will be better insted of return 0.

+	if (test_and_set_bit(BTRFS_FS_EXCL_OP, &fs_info->flags)) {
+		btrfs_info(fs_info,
+		"cannot resume dev-replace, other exclusive operation running");
+		return 0;
+	}


Thanks, Anand

> that is balance will start and
>   replace will hit EXCL_OP and thus canceled? How does it work in
>   this case?
> 
> Thanks, Anand
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