Re: [PATCH v3 4/6] btrfs-progs: lowmem check: Add dev_item check for used bytes and total bytes

[Hans van Kranenburg <Hans.van.Kranenburg@mendix.com>]

On 10/09/2018 03:14 AM, Qu Wenruo wrote:
> 
> 
> On 2018/10/9 上午6:20, Hans van Kranenburg wrote:
>> On 10/08/2018 02:30 PM, Qu Wenruo wrote:
>>> Obviously, used bytes can't be larger than total bytes.
>>>
>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>> ---
>>>  check/mode-lowmem.c | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/check/mode-lowmem.c b/check/mode-lowmem.c
>>> index 07c03cad77af..1173b963b8f3 100644
>>> --- a/check/mode-lowmem.c
>>> +++ b/check/mode-lowmem.c
>>> @@ -4074,6 +4074,11 @@ static int check_dev_item(struct btrfs_fs_info *fs_info,
>>>  	used = btrfs_device_bytes_used(eb, dev_item);
>>>  	total_bytes = btrfs_device_total_bytes(eb, dev_item);
>>>  
>>> +	if (used > total_bytes) {
>>> +		error("device %llu has incorrect used bytes %llu > total bytes %llu",
>>> +			dev_id, used, total_bytes);
>>> +		return ACCOUNTING_MISMATCH;
>>
>> The message and return code point at an error in accounting logic.
> 
> One of the biggest problem in lowmem is we don't always have the error
> code we really want.
> 
> And that's the case for this error message.
> It's indeed not an accounting error, as in that case (just like that
> test case image) the used bytes is correct accounted.
> 
> The good news is, the return value is never really used to classify the
> error.
> So as long as the error message makes sense, it's not a big problem.

Aha. Clear, thanks for explaining.

> 
> Thanks,
> Qu
> 
>>
>> However, if you have a fully allocated device and a DUP chunk ending
>> beyond device, then having used > total_bytes is expected...
>>
>> So maybe there's two possibilities... There's an error in the accounting
>> logic, or there's an "over-allocation", which is another type of issue
>> which produces used > total with correct accounting logic.
>>
>>> +	}
>>>  	key.objectid = dev_id;
>>>  	key.type = BTRFS_DEV_EXTENT_KEY;
>>>  	key.offset = 0;
>>>
>>
>>
> 


-- 
Hans van Kranenburg