Re: [PATCH] scsi: sd_zbc: Disable zoned block devices with scsi-mq

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On 8/17/17 21:19, Damien Le Moal wrote:
> 
> 
> On 8/17/17 16:59, Christoph Hellwig wrote:
>> On Thu, Aug 17, 2017 at 11:45:50AM +0900, Damien Le Moal wrote:
>>> blk-mq may not maintain write requests order at dispatch time. So host
>>> managed drives will not reliably work. Worse, potential reordering of
>>> write requests on requeue may cause the zone write locking code to
>>> deadlock command dispatch to the disk. So for now, until the write
>>> ordering issue is fixed, do not support zoned block devices with
>>> scsi-mq by showing a 0 LBA capacity disk.
>>
>> NAK.  Please fix the issue instead of this.
> 
> I do have a fix ready to send. All the fixes are at blk-mq level and
> basically ensure that dispatch from the hctx->dispatch list are in order
> for write commands (regular writes, write same and write zeroes).
> It is well tested and at least fixes all the problems I have seen with
> dm-zoned and f2fs.
> 
> But:
> 1) It is not small, so I strongly doubt it is RC material
> 2) We need Bart's patch for "always unprepare requests" which was reverted
> 3) Bart does not really like the solution I implemented...
> 
> I can still send it if you want to have a look.

Or were you thinking of just getting the deadlock fixed without
addressing the reordering problem ? In the end, ZBC disks would still
not be usable, so to me, disabling them is exactly equivalent...

-- 
Damien Le Moal,
Western Digital Research



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