On Tue, Nov 28, 2023 at 08:56:37AM +0000, John Garry wrote:

Are you suggesting some sort of hybrid between the atomic write series you 
had a few years ago and this solution?

Very roughly, yes.

To me that would be continuing with the following:
- per-IO RWF_ATOMIC (and not O_ATOMIC semantics of nothing is written until 
some data sync)

Yes.

- writes must be a power-of-two and at a naturally-aligned offset

Where offset is offset in the file?  It would not require it.  You
probably want to do it for optimal performance, but requiring it
feeels rather limited.

- relying on atomic write HW support always

And I think that's where we have different opinions.  I think the hw
offload is a nice optimization and we should use it wherever we can.
But building the entire userspace API around it feels like a mistake.

BTW, we also have rtvol support which does not use forcealign as it already 
can guarantee alignment, but still does rely on the same principle of 
requiring alignment - would you want CoW support there also?

Upstream doesn't have out of place write support for the RT subvolume
yet.  But Darrick has a series for it and we're actively working on
upstreaming it.