Re: [PATCH 1/2] btrfs: drop never met condition of disk_total_bytes == 0

From: Anand Jain <anand.jain@oracle.com>
To: Nikolay Borisov <nborisov@suse.com>, linux-btrfs@vger.kernel.org
Cc: wqu@suse.com, dsterba@suse.com
Subject: Re: [PATCH 1/2] btrfs: drop never met condition of disk_total_bytes == 0
Date: Fri, 25 Sep 2020 12:17:27 +0800	[thread overview]
Message-ID: <bed38208-67ff-ac66-187e-7e8ad91e1968@oracle.com> (raw)
In-Reply-To: <c9e538dd-c039-478c-d677-0e9dd95cfc39@suse.com>

On 24/9/20 7:48 pm, Nikolay Borisov wrote:
> 
> 
> On 24.09.20 г. 13:11 ч., Anand Jain wrote:
>> btrfs_device::disk_total_bytes is set even for a seed device (the
>> comment is wrong).
>>
>> The function fill_device_from_item() does the job of reading it from the
>> item and updating btrfs_device::disk_total_bytes. So both the missing
>> device and the seed devices do have their disk_total_bytes updated.
>>
>> So this patch removes the check dev->disk_total_bytes == 0 in the
>> function verify_one_dev_extent()
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>>   fs/btrfs/volumes.c | 15 ---------------
>>   1 file changed, 15 deletions(-)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index 7f43ed88fffc..9be40eece8ed 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -7578,21 +7578,6 @@ static int verify_one_dev_extent(struct btrfs_fs_info *fs_info,
>>   		goto out;
>>   	}
>>   
>> -	/* It's possible this device is a dummy for seed device */
>> -	if (dev->disk_total_bytes == 0) {
>> -		struct btrfs_fs_devices *devs;
>> -
>> -		devs = list_first_entry(&fs_info->fs_devices->seed_list,
>> -					struct btrfs_fs_devices, seed_list);
>> -		dev = btrfs_find_device(devs, devid, NULL, NULL, false);
>> -		if (!dev) {
>> -			btrfs_err(fs_info, "failed to find seed devid %llu",
>> -				  devid);
>> -			ret = -EUCLEAN;
>> -			goto out;
>> -		}
>> -	}
> 
> The commit which introduced this check states that the device with a
> disk_total_bytes = 0 occurs from clone_fs_devices called from open_seed.
> It seems the check is legit and your changelog doesn't account for that
> if it's safe you should provide description why is that.

yes the commit 1b3922a8bc74 (btrfs: Use real device structure to verify 
dev extent) introduced it. In Qu's analysis, it is unclear why the 
total_disk_bytes was 0.

Theoretically, all devices (including missing and seed) marked with the 
BTRFS_DEV_STATE_IN_FS_METADATA flag gets the total_disk_bytes updated at 
fill_device_from_item().

open_ctree()
  btrfs_read_chunk_tree()
   read_one_dev()
    open_seed_device()
    fill_device_from_item()

Even if verify_one_dev_extent() reports total_disk_bytes == 0, then IMO 
its a bug to be fixed somewhere else and not in verify_one_dev_extent() 
as it's just a messenger. It is never expected that a total_disk_bytes 
shall be zero.

So it is fine to drop the total_disk_bytes == 0 check.

Thanks, Anand

> 
>> -
>>   	if (physical_offset + physical_len > dev->disk_total_bytes) {
>>   		btrfs_err(fs_info,
>>   "dev extent devid %llu physical offset %llu len %llu is beyond device boundary %llu",
>>