Re: [PATCH v7 4/9] block: Add core atomic write support

John Garry
2024-06-03T13:30:06Z

On 03/06/2024 13:31, Hannes Reinecke wrote:
>>
>> It seems ok in principle - we would just need to ensure that it is
>> watertight.
>>
>
> We currently use chunk_sectors for quite some different things, most
> notably zones boundaries, NIOIB, raid stripes etc.
> So I don't have an issue adding another use-case for it.
>
>>> Q2: If we don't, shouldn't we align the atomic write boundary to the
>>> chunk_sectors setting to ensure both match up?
>>
>> Yeah, right. But we can only handle what HW tells.
>>
>> The atomic write boundary is only relevant to NVMe. NVMe NOIOB - which
>> we use to set chunk_sectors - is an IO optimization hint, AFAIK.
>> However the atomic write boundary is a hard limit. So if NOIOB is not
>> aligned with the atomic write boundary - which seems unlikely - then
>> the atomic write boundary takes priority.
>>
> Which is what I said; we need to check. And I would treat a NOIOB value
> not aligned to the atomic write boundary as an error.

Yeah, maybe we can reject that in blk_validate_limits(), by error'ing or
disabling atomic writes there.

>
> But the real issue here is that the atomic write boundary only matters
> for requests, and not for the entire queue.
> So using chunk_sectors is out of question as this would affect all
> requests, and my comment was actually wrong.
> I'll retract it.

I think that some of the logic could be re-used.
rq_straddles_atomic_write_boundary() is checked in merging of reqs/bios
(to see if the resultant req straddles a boundary).

So instead of saying: "will the resultant req straddle a boundary",
re-using path like blk_rq_get_max_sectors() -> blk_chunk_sectors_left(),
we check "is there space within the boundary limit to add this req/bio".
We need to take care of front and back merges, though.

Thanks,
John