Re: [PATCH 4/4] btrfs: show temp_fsid feature in sysfs

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

On 10/6/23 22:55, David Sterba wrote:
> On Wed, Oct 04, 2023 at 11:00:27PM +0800, Anand Jain wrote:
>> This adds sysfs objects to indicate temp_fsid feature support and
>> its status.
>>
>>    /sys/fs/btrfs/features/temp_fsid
>>    /sys/fs/btrfs/<UUID>/temp_fsid
>>
>>    For example:
>>
>>       Consider two cloned and mounted devices.
>>
>> 	$ blkid /dev/sdc[1-2]
>> 	/dev/sdc1: UUID="509ad44b-ad2a-4a8a-bc8d-fe69db7220d5" ..
>> 	/dev/sdc2: UUID="509ad44b-ad2a-4a8a-bc8d-fe69db7220d5" ..
>>
>>       One gets actual fsid, and the other gets the temp_fsid when
>>       mounted.
>>
>> 	$ btrfs filesystem show -m
>> 	Label: none  uuid: 509ad44b-ad2a-4a8a-bc8d-fe69db7220d5
>> 		Total devices 1 FS bytes used 54.14MiB
>> 		devid    1 size 300.00MiB used 144.00MiB path /dev/sdc1
>>
>> 	Label: none  uuid: 33bad74e-c91b-43a5-aef8-b3cab97ae63a
>> 		Total devices 1 FS bytes used 54.14MiB
>> 		devid    1 size 300.00MiB used 144.00MiB path /dev/sdc2
>>
>>       Their sysfs as below.
>>
>> 	$ cat /sys/fs/btrfs/features/temp_fsid
>> 	0
>>
>> 	$ cat /sys/fs/btrfs/509ad44b-ad2a-4a8a-bc8d-fe69db7220d5/temp_fsid
>> 	0
>>
>> 	$ cat /sys/fs/btrfs/33bad74e-c91b-43a5-aef8-b3cab97ae63a/temp_fsid
>> 	1
> 
> So the fsid used for the directory is always the new one, is there a way
> to read which is the original filesystem's fsid? In this case it would
> be the 509ad44b-... We could print it in that file instead of '1',
> though it could be confusing that it's not the temp_fsid but the
> original one, file name mismatches the contents on first look.

Instead, can we emit 'fsid' in another kobject altogether?
Furthermore, we also have a 'metadata_uuid' kobject. Here
is how they relate.


1. normally:

  $ cat /sys/fs/btrfs/<meta-fsid>/fsid
  <meta-fsid>
  $ cat /sys/fs/btrfs/<meta-fsid>/metadata_uuid
  <meta-fsid>


2. metadata-uuid flag is set:

  $ cat /sys/fs/btrfs/<sb-fsid>/fsid
  <sb-fsid>
  $ cat /sys/fs/btrfs/<sb-fsid>/metadata_uuid
  <meta-fsid>


3. normal + temp-fsid:

  $ cat /sys/fs/btrfs/<temp-fsid>/fsid
  <meta-fsid>
  $ cat /sys/fs/btrfs/<temp-fsid>/metadata_uuid
  <meta-fsid>


4. metadata-uuid flag is set + temp-fsid:

  $ cat /sys/fs/btrfs/<temp-fsid>/fsid
  <sb-fsid>
  $ cat /sys/fs/btrfs/<temp-fsid>/metadata_uuid
  <meta-fsid>

> 
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>>   fs/btrfs/sysfs.c | 20 ++++++++++++++++++++
>>   1 file changed, 20 insertions(+)
>>
>> diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
>> index e07be193323a..7f9a4790e013 100644
>> --- a/fs/btrfs/sysfs.c
>> +++ b/fs/btrfs/sysfs.c
>> @@ -425,6 +425,15 @@ static ssize_t acl_show(struct kobject *kobj, struct kobj_attribute *a, char *bu
>>   }
>>   BTRFS_ATTR(static_feature, acl, acl_show);
>>   
>> +static ssize_t temp_fsid_supported_show(struct kobject *kobj,
>> +					struct kobj_attribute *a, char *buf)
>> +{
>> +	int ret = 0;
>> +
>> +	return sysfs_emit(buf, "%d\n", ret);
> 
> This can be
> 
> 	return sysfs_emit(buf, "0\n");

Changes in the misc-next looks better.

Thanks, Anand