[dm-devel] [PATCH] Don't lose writes if errors are not handled and log fails

[Takahiro Yasui]

On 02/02/10 08:00, Mikulas Patocka wrote:
> On Mon, 1 Feb 2010, Takahiro Yasui wrote:
> 
>>> This fixes bug https://bugzilla.redhat.com/show_bug.cgi?id=555197
>>>
>>> Please submit this patch before 2.6.33 goes out. It fixes a bug when old 
>>> LVM (<= 2.02.51) is used, that doesn't pass errors_handled flag to 
>>> dm-raid1.
>>>
>>> It doesn't need to be backported to RHEL 5.5, because lvm always passes a 
>>> flag to handle errors there.
>>
>> Don't we need to backport it to RHEL 5.5? If lvm is the only user of dm-raid1,
>> we don't need to backport it to RHEL 5.5. But if not, we need to.
>>
>>> Don't lose writes if errors are not handled and log fails
>>>
>>> If the log fails and errors are not handled by dmeventd, the writes
>>> are successfully finished without being actually written to the device.
>>>
>>> This code path is taken:
>>> do_writes:
>>> 	bio_list_merge(&ms->failures, &sync);
>>> do_failures:
>>> 	if (!get_valid_mirror(ms)) (false)
>>> 	else if (errors_handled(ms)) (false)
>>> 	else bio_endio(bio, 0);
>>>
>>> The logic in do_failures is based on presuming that the write was already
>>> tried --- if it succeeded at least on one leg and errors are not handled,
>>> it is reported as success.
>>>
>>> However, bio can be added to the failures queue without being submitted, in
>>> do_writes.
>>>
>>> This patch changes it so that bios are added to the failures list only if
>>> errors are handled --- then, they will be held with hold_bio() called from
>>> do_failures.
>>
>> I agree that bios should be issued by do_write() when ms->log_failures is set,
>> but do we need to add bios to the failures queue? As you mentioned, the failures
>> queue should be used to bios which are already handled. Therefore, I think
>> bios are better to handled directly by hold_bio() instead of adding them to
>> the failures queue as bios for nosync regions are done.
>>
>> -	if (unlikely(ms->log_failure)) {
>> -		spin_lock_irq(&ms->lock);
>> -		bio_list_merge(&ms->failures, &sync);
>> -		spin_unlock_irq(&ms->lock);
>> -		wakeup_mirrord(ms);
>> -	} else
>> -		while ((bio = bio_list_pop(&sync)))
>> +	while ((bio = bio_list_pop(&sync)))
>> +		if (unlikely(ms->log_failure) && errors_handled(ms))
>> +			hold_bio(ms, bio);
>> +		else
>>  			do_write(ms, bio);
> 
> I thought about this too, but I'd decided to put the bios on the failures 
> queue rather than holding them for this reason: if all the legs fail, it 
> is better to terminate the bio with -EIO than to hold it. If all the legs 
> fail, you can't save anything anyway and the less things you are holding, 
> the less possibility for deadlocks exists.

Thank you for the explanation. With your following patch, we can keep
consistency of the policy for sync and nosync region.

https://www.redhat.com/archives/dm-devel/2010-February/msg00014.html

Reviewed-by: Takahiro Yasui <tyasui at redhat.com>

Thanks,
Taka


> So I put the bios to the failure queue and do_failures will terminate them 
> with -EIO if all the legs failred and hold them if we use dmeventd and 
> there is at least one live leg.
>> https://www.redhat.com/archives/dm-devel/2009-December/msg00211.html
>>
>> Thanks,
>> Taka
>>