Re: [PATCH 21/21] nvme: Support atomic writes

[John Garry]

On 10/11/2023 06:29, Christoph Hellwig wrote:
> Yes.
>
> > As for splitting, it is not permitted for atomic writes and only a single
> > bio is permitted to be created per write. Are more integrity checks
> > required?
> I'm more worried about the problem where we accidentally add a split.
> The whole bio merge/split path is convoluted and we had plenty of
> bugs in the past by not looking at all the correct flags or opcodes.

Yes, this is always a concern.

Some thoughts on things which could be done:
- For no merging, ensure request length is a power-of-2 when enqueuing 
to block driver. This is simple but not watertight.
- Create a per-bio checksum when the bio is created for the atomic write 
and ensure integrity when queuing to the block driver
- a new block layer datapath which ensures no merging or splitting, but 
this seems a bit OTT

BTW, on topic of splitting, that NVMe virt boundary is a pain and I hope 
that we could ignore/avoid it for atomic writes.

Thanks,
John