Re: [PATCH v2] Btrfs: avoid losing data raid profile when deleting a device

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

On 10/14/2017 04:51 AM, Liu Bo wrote:
> On Wed, Oct 11, 2017 at 10:38:50AM +0300, Nikolay Borisov wrote:
>>
>>
>> On 10.10.2017 20:53, Liu Bo wrote:
>>> We've avoided data losing raid profile when doing balance, but it
>>> turns out that deleting a device could also result in the same
>>> problem
>>>
>>> This fixes the problem by creating an empty data chunk before
>>> relocating the data chunk.
>>
>> Why is this needed - copy the metadata of the to-be-relocated chunk into
>> the newly created empty chunk? I don't entirely understand that code but
>> doesn't this seem a bit like a hack in order to stash some information?
>> Perhaps you could elaborate the logic a bit more in the changelog?
>>
>>>
>>> Metadata/System chunk are supposed to have non-zero bytes all the time
>>> so their raid profile is persistent.
>>
>> I think this changelog is a bit scarce on detail as to the culprit of
>> the problem. Could you perhaps put a sentence or two what the underlying
>> logic which deletes the raid profile if a chunk is empty ?
>>
> 
> Fair enough.
> 
> The problem is as same as what commit 2c9fe8355258 ("btrfs: Fix
> lost-data-profile caused by balance bg") had fixed.
> 
> Similar to doing balance, deleting a device can also move all chunks
> on this disk to other available disks, after 'move' successfully,
> it'll remove those chunks.
> 
> If our last data chunk is empty and part of it happens to be on this
> disk, then there is no data chunk in this btrfs after deleting the
> device successfully, any following write will try to create a new data
> chunk which ends up with a single data chunk because the only
> available data raid profile is 'single'.

  So you are referring to a raid1 group profile which contains 3 or more
  devices otherwise single group file is what it will fit ? Is there
  reproducer ?

Thanks, Anand


> thanks,
> -liubo
> 
>>>
>>> Reported-by: James Alandt <James.Alandt@wdc.com>
>>> Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
>>> ---
>>>
>>> v2: - return the correct error.
>>>      - move helper ahead of __btrfs_balance().
>>>
>>>   fs/btrfs/volumes.c | 84 ++++++++++++++++++++++++++++++++++++++++++------------
>>>   1 file changed, 65 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>>> index 4a72c45..a74396d 100644
>>> --- a/fs/btrfs/volumes.c
>>> +++ b/fs/btrfs/volumes.c
>>> @@ -3018,6 +3018,48 @@ static int btrfs_relocate_sys_chunks(struct btrfs_fs_info *fs_info)
>>>   	return ret;
>>>   }
>>>   
>>> +/*
>>> + * return 1 : allocate a data chunk successfully,
>>> + * return <0: errors during allocating a data chunk,
>>> + * return 0 : no need to allocate a data chunk.
>>> + */
>>> +static int btrfs_may_alloc_data_chunk(struct btrfs_fs_info *fs_info,
>>> +				      u64 chunk_offset)
>>> +{
>>> +	struct btrfs_block_group_cache *cache;
>>> +	u64 bytes_used;
>>> +	u64 chunk_type;
>>> +
>>> +	cache = btrfs_lookup_block_group(fs_info, chunk_offset);
>>> +	ASSERT(cache);
>>> +	chunk_type = cache->flags;
>>> +	btrfs_put_block_group(cache);
>>> +
>>> +	if (chunk_type & BTRFS_BLOCK_GROUP_DATA) {
>>> +		spin_lock(&fs_info->data_sinfo->lock);
>>> +		bytes_used = fs_info->data_sinfo->bytes_used;
>>> +		spin_unlock(&fs_info->data_sinfo->lock);
>>> +
>>> +		if (!bytes_used) {
>>> +			struct btrfs_trans_handle *trans;
>>> +			int ret;
>>> +
>>> +			trans =	btrfs_join_transaction(fs_info->tree_root);
>>> +			if (IS_ERR(trans))
>>> +				return PTR_ERR(trans);
>>> +
>>> +			ret = btrfs_force_chunk_alloc(trans, fs_info,
>>> +						      BTRFS_BLOCK_GROUP_DATA);
>>> +			btrfs_end_transaction(trans);
>>> +			if (ret < 0)
>>> +				return ret;
>>> +
>>> +			return 1;
>>> +		}
>>> +	}
>>> +	return 0;
>>> +}
>>> +
>>>   static int insert_balance_item(struct btrfs_fs_info *fs_info,
>>>   			       struct btrfs_balance_control *bctl)
>>>   {
>>> @@ -3476,7 +3518,6 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
>>>   	u32 count_meta = 0;
>>>   	u32 count_sys = 0;
>>>   	int chunk_reserved = 0;
>>> -	u64 bytes_used = 0;
>>>   
>>>   	/* step one make some room on all the devices */
>>>   	devices = &fs_info->fs_devices->devices;
>>> @@ -3635,28 +3676,21 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
>>>   			goto loop;
>>>   		}
>>>   
>>> -		ASSERT(fs_info->data_sinfo);
>>> -		spin_lock(&fs_info->data_sinfo->lock);
>>> -		bytes_used = fs_info->data_sinfo->bytes_used;
>>> -		spin_unlock(&fs_info->data_sinfo->lock);
>>> -
>>> -		if ((chunk_type & BTRFS_BLOCK_GROUP_DATA) &&
>>> -		    !chunk_reserved && !bytes_used) {
>>> -			trans = btrfs_start_transaction(chunk_root, 0);
>>> -			if (IS_ERR(trans)) {
>>> -				mutex_unlock(&fs_info->delete_unused_bgs_mutex);
>>> -				ret = PTR_ERR(trans);
>>> -				goto error;
>>> -			}
>>> -
>>> -			ret = btrfs_force_chunk_alloc(trans, fs_info,
>>> -						      BTRFS_BLOCK_GROUP_DATA);
>>> -			btrfs_end_transaction(trans);
>>> +		if (!chunk_reserved) {
>>> +			/*
>>> +			 * We may be relocating the only data chunk we have,
>>> +			 * which could potentially end up with losing data's
>>> +			 * raid profile, so lets allocate an empty one in
>>> +			 * advance.
>>> +			 */
>>> +			ret = btrfs_may_alloc_data_chunk(fs_info,
>>> +							 found_key.offset);
>>>   			if (ret < 0) {
>>>   				mutex_unlock(&fs_info->delete_unused_bgs_mutex);
>>>   				goto error;
>>> +			} else if (ret == 1) {
>>> +				chunk_reserved = 1;
>>>   			}
>>> -			chunk_reserved = 1;
>>>   		}
>>>   
>>>   		ret = btrfs_relocate_chunk(fs_info, found_key.offset);
>>> @@ -4419,6 +4453,18 @@ int btrfs_shrink_device(struct btrfs_device *device, u64 new_size)
>>>   		chunk_offset = btrfs_dev_extent_chunk_offset(l, dev_extent);
>>>   		btrfs_release_path(path);
>>>   
>>> +		/*
>>> +		 * We may be relocating the only data chunk we have,
>>> +		 * which could potentially end up with losing data's
>>> +		 * raid profile, so lets allocate an empty one in
>>> +		 * advance.
>>> +		 */
>>> +		ret = btrfs_may_alloc_data_chunk(fs_info, chunk_offset);
>>> +		if (ret < 0) {
>>> +			mutex_unlock(&fs_info->delete_unused_bgs_mutex);
>>> +			goto done;
>>> +		}
>>> +
>>>   		ret = btrfs_relocate_chunk(fs_info, chunk_offset);
>>>   		mutex_unlock(&fs_info->delete_unused_bgs_mutex);
>>>   		if (ret && ret != -ENOSPC)
>>>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>