On 2018/10/11 下午9:11, Nikolay Borisov wrote:
>
>
> On 11.10.2018 15:45, Qu Wenruo wrote:
>>
>>
>> On 2018/10/11 下午8:31, David Sterba wrote:
>>> On Thu, Oct 04, 2018 at 01:49:49PM +0800, Qu Wenruo wrote:
>>>> We have btrfs_super_block::log_root_transid to record the transid of log
>>>> tree root.
>>>>
>>>> However it's never populated and it's always 0, just as the following
>>>> dump-super:
>>>> log_root 30572544
>>>> log_root_transid 0
>>>> log_root_level 0
>>>>
>>>> This patch will populate it with log tree root correctly, so the result
>>>> will be:
>>>> log_root 30572544
>>>> log_root_transid 6
>>>> log_root_level 0
>>>>
>>>> This won't affect current kernel behavior or btrfs check result as we
>>>> already expect log tree root generation always to be super block
>>>> genration + 1.
>>>>
>>>> But it could be later used to detect log tree corruption early.
>>>
>>> The backward compatibility seems to be ok in general, I found one
>>> scenario where the check will fail:
>>>
>>> * mount with unpatched kernel, log_root_transid = 0
>>> * mount with patched kernel, log_root_transid 100, generation 101
>>> * mount with unpatched kernel, log_root_transid 100 (unchanged), generation
>>> 201
>>> * mount with patched kernel -> check fails
>>
>> Indeed, this is a problem I missed.
>>
>> It provides the old principle, if we're going to change how kernel
>> use/interprete a on-disk memeber, we have to introduce a new
>> incompatible flag.
>>
>> Please just drop the series of patch.
>
> Since this member really doesn't have any purpose now I think the
> prudent thing to do is to rename it to "reserved1" or "padding" or
> whatever but basically signal that it's no longer in used. In the future
> when a new incompaat bit will be required for some feature it can be
> re-used.
Makes sense.
I'll craft a patch for that.
Thanks,
Qu
>
>>
>> Thanks,
>> Qu
>>>
>>> So the problem is when log_root_transid is not 0 and changes out of sync
>>> with the generation. The above sequence of kernels can simply happen
>>> when switching between old stable and current releases.
>>>
>>
