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

From: John Garry
Date: Mon Jun 03 2024 - 09:30:26 EST


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