[dm-devel] dm-zoned: Avoid metadata flush writeback throttling

[Damien Le Moal]

Mike,

On 9/12/17 00:05, Mike Snitzer wrote:
> On Mon, Jul 24 2017 at  5:16am -0400,
> Damien Le Moal <damien.lemoal at wdc.com> wrote:
> 
>>
>>
>> On 7/24/17 17:03, Christoph Hellwig wrote:
>>> On Mon, Jul 24, 2017 at 04:44:22PM +0900, Damien Le Moal wrote:
>>>> Avoid metadata flush to be blocked by the writeback throttling code in
>>>> wbt_wait(). Otherwise, we can end up with a deadlock under heavy write
>>>> load with all chunk works active. In such situation, a deadlock can
>>>> happen if the flush work is blocked by the throttling code while holding
>>>> the metadata lock: as the chunk works will wait for the metadata lock
>>>> too, no progress can be made.
>>>>
>>>> Signed-off-by: Damien Le Moal <damien.lemoal at wdc.com>
>>>> ---
>>>>  drivers/md/dm-zoned-metadata.c | 3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/md/dm-zoned-metadata.c b/drivers/md/dm-zoned-metadata.c
>>>> index 884ff7c170a0..f694fb98b002 100644
>>>> --- a/drivers/md/dm-zoned-metadata.c
>>>> +++ b/drivers/md/dm-zoned-metadata.c
>>>> @@ -567,7 +567,8 @@ static void dmz_write_mblock(struct dmz_metadata *zmd, struct dmz_mblock *mblk,
>>>>  	bio->bi_bdev = zmd->dev->bdev;
>>>>  	bio->bi_private = mblk;
>>>>  	bio->bi_end_io = dmz_mblock_bio_end_io;
>>>> -	bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_META | REQ_PRIO);
>>>> +	bio_set_op_attrs(bio, REQ_OP_WRITE,
>>>> +			 REQ_META | REQ_PRIO | REQ_SYNC | REQ_IDLE);
>>>
>>> Please just assign bi_opf directly instea dof using bio_set_op_attrs
>>> in new code.
>>
>> OK. Will do.
>>
>>> Also what do you need REQ_IDLE for?
>>
>> It is not really needed here, but the wbt_should_throttle() code test is:
>>
>> if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) == (REQ_SYNC | REQ_IDLE))
>> 	return false;
>>
>> So if REQ_IDLE is not set, wbt will trigger.
>>
>> Having a req flag that says "no wb throttling please" would be a lot
>> cleaner...
> 
> Did you have a new version of this patch?
> 
> Thanks,
> Mike

My apologies, I forgot about this one. Sending right away.

Best regards.

-- 
Damien Le Moal,
Western Digital