Re: [PATCH] scsi: ufs: mark HPB support as BROKEN

[Ming Lei <ming.lei@redhat.com>]

On Wed, Oct 27, 2021 at 09:06:05AM -0600, Jens Axboe wrote:
> On 10/27/21 9:03 AM, Ming Lei wrote:
> > On Wed, Oct 27, 2021 at 07:12:31AM -0700, Keith Busch wrote:
> >> On Wed, Oct 27, 2021 at 06:16:19AM -0700, Bart Van Assche wrote:
> >>> On 10/26/21 10:27 PM, Christoph Hellwig wrote:
> >>>> On Tue, Oct 26, 2021 at 01:10:47PM -0700, Bart Van Assche wrote:
> >>>>> If blk_insert_cloned_request() is moved into the device mapper then I
> >>>>> think that blk_mq_request_issue_directly() will need to be exported.
> >>>>
> >>>> Which is even worse.
> >>>>
> >>>>> How
> >>>>> about the (totally untested) patch below for removing the
> >>>>> blk_insert_cloned_request() call from the UFS-HPB code?
> >>>>
> >>>> Which again doesn't fix anything.  The problem is that it fans out one
> >>>> request into two on the same queue, not the specific interface used.
> >>>
> >>> That patch fixes the reported issue, namely removing the additional accounting
> >>> caused by calling blk_insert_cloned_request(). Please explain why it is
> >>> considered wrong to fan out one request into two. That code could be reworked
> >>> such that the block layer is not involved as Adrian Hunter explained. However,
> >>> before someone spends time on making these changes I think that someone should
> >>> provide more information about why it is considered wrong to fan out one request
> >>> into two.
> >>
> >> The original request consumes a tag from that queue's tagset. If the
> >> lifetime of that tag depends on that same queue having another free tag,
> >> you can deadlock.
> > 
> > Just take a quick look at the code, if the spawned request can't be allocated,
> > scsi will return BLK_STS_RESOURCE for the original scsi request which will be
> > retried later by blk-mq.
> > 
> > So if tag depth is > 1 and max allowed inflight write buffer command is limited
> > as 1, there shouldn't be the deadlock.
> > 
> > Or is it possible to reuse the original scsi request's tag for the
> > spawned request? Like the trick used in inserting flush request.
> 
> The flush approach did come to mind here as well, but honestly that one is
> very ugly and would never have been permitted if it wasn't excluded to be
> in the very core code already. But yes, reuse of the existing request is
> probably another potentially viable approach. My worry there is that
> inevitably you end up needing to stash a lot of data to restore the original,
> and we're certainly not adding anything to struct request for that.
> 
> Hence I think being able to find a new request reliably would be better.

request with scsi_cmnd may be allocated by the ufshpb driver, even it
should be fine to call ufshcd_queuecommand() directly for this driver
private IO, if the tag can be reused. One example is scsi_ioctl_reset().


Thanks,
Ming