Note that the alignment rule is not just for atomic HW boundaries. We
also support atomic writes on stacked devices, where this is relevant -
specifically striped devices, like raid0. Doing an unaligned atomic
write on a striped device may result in trying to issue an atomic write
which straddles 2x separate devices, which would obviously be broken.

Ok, then I'd also add atomic boundary checks and
atomic_write_boundary_bytes = stripe size to /sys/block/**/queue for
md devices. Not 2^N and length-alignment checks, just the boundary.

It seems that you just want to take advantage of the block layer code to
handle submission of an atomic write bio, i.e. reject anything which
cannot be atomically written. In essence, that would be to just set
REQ_ATOMIC. Maybe that could be done as a passthrough command, I'm not sure.

Of course. I thought it was the whole point of RWF_ATOMIC - REQ_ATOMIC
for userspace.

I don't want passthrough commands, I want to use normal kernel I/O.