Re: [PATCH v3 10/15] block: Add fops atomic write support

[John Garry]

On 13/02/2024 11:08, Nilay Shroff wrote:
> > It's relied that atomic_write_unit_max is <= atomic_write_boundary and both are a power-of-2. Please see the NVMe patch, which this is checked. Indeed, it would not make sense if atomic_write_unit_max > atomic_write_boundary (when non-zero).
> >
> > So if the write is naturally aligned and its size is <= atomic_write_unit_max, then it cannot be straddling a boundary.
> Ok fine but in case the device doesn't support namespace atomic boundary size (i.e. NABSPF is zero) then still do we need
> to restrict IO which crosses the atomic boundary?

Is there a boundary if NABSPF is zero?

> I am quoting this from NVMe spec (Command Set Specification, revision 1.0a, Section 2.1.4.3) :
> "To ensure backwards compatibility, the values reported for AWUN, AWUPF, and ACWU shall be set such that
> they  are  supported  even  if  a  write  crosses  an  atomic  boundary.  If  a  controller  does  not
> guarantee atomicity across atomic boundaries, the controller shall set AWUN, AWUPF, and ACWU to 0h (1 LBA)."

How about respond to the NVMe patch in this series, asking this question?

I have my idea on how the boundary is determined, but I think that the 
spec could be made clearer.

Thanks,
John