[Ocfs2-devel] [PATCH v2] ocfs2: check the metadate alloc before marking extent written

[Changwei Ge]

On 2017/12/13 15:25, alex chen wrote:
> Hi Changwei,
> 
> On 2017/12/12 9:05, Changwei Ge wrote:
>> Hi Alex,
>>
>> On 2017/12/5 11:31, alex chen wrote:
>>> We need to check the free number of the records in each loop to mark
>>> extent written, because the last extent block may be changed through
>>> many times marking extent written and the 'num_free_extents' also be
>>> changed. In the worst case, the 'num_free_extents' may become less
>>> than the beginning of the loop. So we should not estimate the free
>>> number of the records at the beginning of the loop to mark extent
>>> written.
>>>
>>> Reported-by: John Lightsey <john at nixnuts.net>
>>> Signed-off-by: Alex Chen <alex.chen at huawei.com>
>>> Reviewed-by: Jun Piao <piaojun at huawei.com>
>>> ---
>>>    fs/ocfs2/aops.c | 77 +++++++++++++++++++++++++++++++++++++++++++++++----------
>>>    1 file changed, 64 insertions(+), 13 deletions(-)
>>>

>>
>> If there isn't enough extent records here, you break this loop and also
>> commit transaction.
>> After re-executing from _restart_all_, a new transaction is started.
>> So you separate 'before _restart_all_' and 'after _restart_all_' into
>> different transactions.
>>
>>
>>>
>>> -	if (end > i_size_read(inode)) {
>>> +	if (!restart && end > i_size_read(inode)) {
>>
>> Here you don't change inode size, however, obviously you already mark
>> extents as *written*.
> 
> Here we should not change inode size, otherwise, the user may read some of the data
> he wrote and the other part of data is zero, which breaks the consistency of WRITE.
> Here it just mark extents to written and do not allocated data spaces.

Very true.

> 
>>
>>>    		ret = ocfs2_set_inode_size(handle, inode, di_bh, end);
>>>    		if (ret < 0)
>>>    			mlog_errno(ret);
>>>    	}
>>> +
>>>    commit:
>>>    	ocfs2_commit_trans(osb, handle);
>>
>> You commit transaction here and host just crashes at this point.
>> Then you will have a file with a smaller size than its real size.
>> Moreover, those space could not be declaimed.
> 
> The problem you described above is exist. But it is a new problem and is not introduced by this patch.
> It is introduced in commit c15471f79506(ocfs2: fix sparse file & data ordering issue in direct io).
> Those space was allocated in ocfs2_dio_wr_get_block()->ocfs2_write_begin_nolock()->
> ocfs2_write_cluster_by_desc() and it could not be declaimed when any errors happen in
> ocfs2_dio_end_io_write().

Hi Alex,
Actually, 1)the space can be declaimed via ocfs2 journal recovery by other nodes or itself during its
next mount, since it also resided in orphan directory. 2)If deleting corresponding inode is done, I suppose,
unwritten cluster might be declaimed by _fsck_ with a *consistent* inode size.(not very sure on this point).

The original version of ocfs2_dio_end_io_write() before your patch being applied, it ties _mark_extent_written_
to _set_inode_size_ in an unique jbd2 transaction. So it can guarantee the consistency between changing them.

As for your patch, you split them into different transactions which breaks the consistency, since your patch
starts and commit a transaction once _restart_all_ is needed(2 times at least), but only the last jbd2 committed
transaction includes changing inode size.

I think your way to fix this issue is acceptable.

If you have no idea how to improve patch, I will try to compose a patch to do some help based on your patch.

Perhaps Andrew can fold them into a single one, it's up to Andrew.

Thanks,
Changwei

> This patch corrects the calculation method of the free number of the records.
> 
> At present, I have no idea to solve the new problem you described above, I think you can send the patch
> to solve this problem if you have a good idea.
> 
> Thanks,
> Alex
> 
>>
>> Should we do more work on JBD2?
>>
>> Thanks,
>> Changwei
>>
>>
>>> +free_ac:
>>> +	if (meta_ac) {
>>> +		ocfs2_free_alloc_context(meta_ac);
>>> +		meta_ac = NULL;
>>> +	}
>>> +	if (restart) {
>>> +		restart = 0;
>>> +		goto restart_all;
>>> +	}
>>>    unlock:
>>>    	up_write(&oi->ip_alloc_sem);

> 
>