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

[Damien Le Moal]

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;

No if REQ_IDLE is not set, wbt will trigger.

Having a req flag that says "no wb throttling please" would be a lot
cleaner...

-- 
Damien Le Moal,
Western Digital