Thanks for writing a good commit message!
NVMe namespaces may define an atomic boundary, whereby no atomic guarantees
are provided for a write which straddles this per-lba space boundary. The
block layer merging policy is such that no merges may occur in which the
resultant request would straddle such a boundary.
Unlike SCSI, NVMe specifies no granularity or alignment rules.
Well, the boundary really is sort of a granularity and alignment,
isn't it?