Re: [PATCH block v2 2/3] block: Add support for REQ_NOZERO flag

Kirill Tkhai
2020-01-21T09:47:31Z

On 21.01.2020 09:14, Martin K. Petersen wrote:
>
> Kirill,
>
>> + if (flags & BLKDEV_ZERO_NOUNMAP)
>> + req_flags |= REQ_NOUNMAP;
>> + if (flags & BLKDEV_ZERO_ALLOCATE)
>> + req_flags |= REQ_NOZERO|REQ_NOUNMAP;
>
> I find there is some dissonance between using BLKDEV_ZERO_ALLOCATE to
> describe this operation in one case and REQ_NOZERO in the other.
>
> I understand why not zeroing is important in your case. However, I think
> the allocation aspect is semantically more important. Also, in the case
> of SCSI, the allocated blocks will typically appear zeroed. So from that
> perspective REQ_NOZERO doesn't really make sense. I would really prefer
> to use REQ_ALLOCATE to describe this operation. I agree that "do not
> write every block" is important too. I just don't have a good suggestion
> for how to express that as an additional qualifier to REQ_ALLOCATE_?.

No problem, I'll rename the modifier.

> Also, adding to the confusion: In the context of SCSI, ANCHOR requires
> UNMAP. So my head hurts a bit when I read REQ_NOZERO|REQ_NOUNMAP and
> have to translate that into ANCHOR|UNMAP.
>
> Longer term, I think we should consider introducing REQ_OP_SINGLE_RANGE
> or something like that as an umbrella operation that can be used to
> describe zeroing, allocating, and other things that operate on a single
> LBA range with no payload. Thus removing both the writiness and the
> zeroness from the existing REQ_OP_WRITE_ZEROES conduit.
>
> Naming issues aside, your patch looks fine. I'll try to rebase my SCSI
> patches on top of your series to see how things fit.

Ok, thanks.