[Ocfs2-devel] [PATCH v2] ocfs2: return error when we attempt to access a dirty bh in jbd2

[piaojun]

Hi Changwei,

On 2018/1/27 19:19, Changwei Ge wrote:
> Hi Jun,
> 
> On 2018/1/27 16:28, piaojun wrote:
>> We should not reuse the dirty bh in jbd2 directly due to the following
>> situation:
>>
>> 1. When removing extent rec, we will dirty the bhs of extent rec and
>>     truncate log at the same time, and hand them over to jbd2.
>> 2. The bhs are submitted to jbd2 area successfully.
>> 3. The write-back thread of device help flush the bhs to disk but
>>     encounter write error due to abnormal storage link.
>> 4. After a while the storage link become normal. Truncate log flush
>>     worker triggered by the next space reclaiming found the dirty bh of
>>     truncate log and clear its 'BH_Write_EIO' and then set it uptodate in
>>     __ocfs2_journal_access():
>>
>> ocfs2_truncate_log_worker
>>    ocfs2_flush_truncate_log
>>      __ocfs2_flush_truncate_log
>>        ocfs2_replay_truncate_records
>>          ocfs2_journal_access_di
>>            __ocfs2_journal_access // here we clear io_error and set 'tl_bh' uptodata.
>>
>> 5. Then jbd2 will flush the bh of truncate log to disk, but the bh of
>>     extent rec is still in error state, and unfortunately nobody will
>>     take care of it.
>> 6. At last the space of extent rec was not reduced, but truncate log
>>     flush worker have given it back to globalalloc. That will cause
>>     duplicate cluster problem which could be identified by fsck.ocfs2.
>>
>> Sadlly we can hardly revert this but set fs read-only in case of
>> ruining atomicity and consistency of space reclaim.
>>
>> Fixes: acf8fdbe6afb ("ocfs2: do not BUG if buffer not uptodate in __ocfs2_journal_access")
>>
>> Signed-off-by: Jun Piao <piaojun at huawei.com>
>> Reviewed-by: Yiwen Jiang <jiangyiwen at huawei.com>
>> ---
>>   fs/ocfs2/journal.c | 49 ++++++++++++++++++++++++++++++++++++++-----------
>>   1 file changed, 38 insertions(+), 11 deletions(-)
>>
>> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
>> index 3630443..4c5661c 100644
>> --- a/fs/ocfs2/journal.c
>> +++ b/fs/ocfs2/journal.c
>> @@ -666,23 +666,50 @@ static int __ocfs2_journal_access(handle_t *handle,
>>   	/* we can safely remove this assertion after testing. */
>>   	if (!buffer_uptodate(bh)) {
>>   		mlog(ML_ERROR, "giving me a buffer that's not uptodate!\n");
>> -		mlog(ML_ERROR, "b_blocknr=%llu\n",
>> -		     (unsigned long long)bh->b_blocknr);
>> +		mlog(ML_ERROR, "b_blocknr=%llu, b_state=0x%lx\n",
>> +		     (unsigned long long)bh->b_blocknr, bh->b_state);
>>
>>   		lock_buffer(bh);
>>   		/*
>> -		 * A previous attempt to write this buffer head failed.
>> -		 * Nothing we can do but to retry the write and hope for
>> -		 * the best.
>> +		 * We should not reuse the dirty bh directly due to the
>> +		 * following situation:
>> +		 *
>> +		 * 1. When removing extent rec, we will dirty the bhs of
>> +		 *    extent rec and truncate log at the same time, and
>> +		 *    hand them over to jbd2.
>> +		 * 2. The bhs are submitted to jbd2 area successfully.
>> +		 * 3. The write-back thread of device help flush the bhs
>> +		 *    to disk but encounter write error due to abnormal
>> +		 *    storage link.
>> +		 * 4. After a while the storage link become normal.
>> +		 *    Truncate log flush worker triggered by the next
>> +		 *    space reclaiming found the dirty bh of truncate log
>> +		 *    and clear its 'BH_Write_EIO' and then set it uptodate
>> +		 *    in __ocfs2_journal_access():
>> +		 *
>> +		 *    ocfs2_truncate_log_worker
>> +		 *      ocfs2_flush_truncate_log
>> +		 *        __ocfs2_flush_truncate_log
>> +		 *          ocfs2_replay_truncate_records
>> +		 *            ocfs2_journal_access_di
>> +		 *              __ocfs2_journal_access
>> +		 *
>> +		 * 5. Then jbd2 will flush the bh of truncate log to disk,
>> +		 *    but the bh of extent rec is still in error state, and
>> +		 *    unfortunately nobody will take care of it.
>> +		 * 6. At last the space of extent rec was not reduced,
>> +		 *    but truncate log flush worker have given it back to
>> +		 *    globalalloc. That will cause duplicate cluster problem
>> +		 *    which could be identified by fsck.ocfs2.
>> +		 *
>> +		 * Sadlly we can hardly revert this but set fs read-only
>> +		 * in case of ruining atomicity and consistency of space
>> +		 * reclaim.
>>   		 */
> 
> It's tons of comments here and it seems the same with what's in change log.
> Must we add them here?
> Besides, the scenario the comment is describing is not a common case.
> I think the issue will also cause some other metadata inconsistency.
> So the comments will make maintainers puzzle afterwards.
> I suggest to simplify it like below, for your reference:
> 
> A previous transaction with a couple buffer heads fails checkpoint, so all those buffer heads are marked IO_ERROR.
> For current transaction, a buffer head is shared with the previous one with IO_ERROR.
> We can't just clear IO_ERROR and continue ,since other buffer heads are not written to disk yet.
> Above case will cause metadata inconsistency.
> Just return -EORFS here and abort journal to avoid damage file system.
> 
> Thanks,
> Changwei
> 
Your suggestion makes sense, and I will simplify it later in patch v3.

thanks,
Jun

>>   		if (buffer_write_io_error(bh) && !buffer_uptodate(bh)) {
>> -			clear_buffer_write_io_error(bh);
>> -			set_buffer_uptodate(bh);
>> -		}
>> -
>> -		if (!buffer_uptodate(bh)) {
>>   			unlock_buffer(bh);
>> -			return -EIO;
>> +			return ocfs2_error(osb->sb, "A previous attempt to "
>> +					"write this buffer head failed\n");
>>   		}
>>   		unlock_buffer(bh);
>>   	}
>>
> .
>