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

[Christoph Hellwig]

On Thu, Nov 09, 2023 at 07:08:40PM +0000, John Garry wrote:
>>> send a write that crossed the atomic write limit, but the drive wouldn't
>>> guarantee that it was atomic except at the atomic write boundary.
>>> Eg with an AWUN of 16kB, you could send five 16kB writes, combine them
>>> into a single 80kB write, and if the power failed midway through, the
>>> drive would guarantee that it had written 0, 16kB, 32kB, 48kB, 64kB or
>>> all 80kB.  Not necessarily in order; it might have written bytes 16-32kB,
>>> 64-80kB and not the other three.
>
> I didn't think that there are any atomic write guarantees at all if we ever 
> exceed AWUN or AWUPF or cross the atomic write boundary (if any).

You're quoting a few mails before me, but I agree.

>> I can see some use for that, but I'm really worried that debugging
>> problems in the I/O merging and splitting will be absolute hell.
>
> Even if bios were merged for NVMe the total request length still should not 
> exceed AWUPF. However a check can be added to ensure this for a submitted 
> atomic write request.

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.