On Mar 16, 2014, at 3:24 PM, Andreas Rohner wrote:
>>>
>>> diff --git a/include/linux/nilfs2_fs.h b/include/linux/nilfs2_fs.h
>>> index ff3fea3..ca269ad 100644
>>> --- a/include/linux/nilfs2_fs.h
>>> +++ b/include/linux/nilfs2_fs.h
>>> @@ -614,11 +614,13 @@ struct nilfs_cpfile_header {
>>> * @su_lastmod: last modified timestamp
>>> * @su_nblocks: number of blocks in segment
>>> * @su_flags: flags
>>> + * @su_lastdec: last decrement of su_nblocks timestamp
>>> */
>>> struct nilfs_segment_usage {
>>> __le64 su_lastmod;
>>> __le32 su_nblocks;
>>> __le32 su_flags;
>>> + __le64 su_lastdec;
>>
>> So, this change makes on-disk layout incompatible with previous one.
>> Am I correct? At first it needs to be fully confident that we really need in
>> changing in this place. Secondly, it needs to add incompatible flag for
>> s_feature_incompat field of superblock and maybe mount option.
>
> No it IS compatible. NILFS uses the entry sizes stored in the super
> block. Notice, that the code does not depend on sizeof(struct
> nilfs_suinfo) or sizeof(struct nilfs_segment_usage). So an old kernel
> can read a file system with su_lastdec and a new kernel can read an old
> file system without su_lastdec.
But, anyway, I think that you add some new feature by this and previous
patches. I suppose that it makes sense to add specially dedicated flag or
flags in s_feature_xxx field of superblock. If feature is compatible with
previous state of driver then flag can be added for s_feature_compat
field.
Thanks,
Vyacheslav Dubeyko.
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