Re: Regression: NVMe: kernel BUG at lib/sg_pool.c:103!

["Martin K. Petersen" <martin.petersen@oracle.com>]

Christoph,

>> 1. We are using RQF_SPECIAL_PAYLOAD for only discard commands and not
>> for write-zeroes because it does not have any payload. Using this in
>> the code will trigger more code changes to handle in the completion
>> path.
>
> Yes.  And that is the big difference to SCSI where REQ_OP_WRITE_ZEROES
> turns into a WRITE SAME command that has a payload.  So for SCSI
> RQF_SPECIAL_PAYLOAD for REQ_OP_WRITE_ZEROES makes a lot of sense, for
> NVMe it does not.

I don't actually care about using RQF_SPECIAL, it just seemed like a
quick workaround to set it and make bv_len 0.

My concern is purely rooted in all the grief we've had throughout the
block I/O stack distinguishing between the bytes acted upon on media and
the DMA transfer length. And consequently, I don't particularly like
that blk_rq_payload_bytes() doesn't handle the NVMe WRITE ZEROES
command. That seems like something that will cause us headaches in the
future...

-- 
Martin K. Petersen	Oracle Linux Engineering