Re: [PATCH v4 10/11] nvme: Atomic write support

[Keith Busch]

On Mon, Feb 19, 2024 at 01:01:08PM +0000, John Garry wrote:
> From: Alan Adamson <alan.adamson@xxxxxxxxxx>
> 
> Add support to set block layer request_queue atomic write limits. The
> limits will be derived from either the namespace or controller atomic
> parameters.
> 
> NVMe atomic-related parameters are grouped into "normal" and "power-fail"
> (or PF) class of parameter. For atomic write support, only PF parameters
> are of interest. The "normal" parameters are concerned with racing reads
> and writes (which also applies to PF). See NVM Command Set Specification
> Revision 1.0d section 2.1.4 for reference.
> 
> Whether to use per namespace or controller atomic parameters is decided by
> NSFEAT bit 1 - see Figure 97: Identify - Identify Namespace Data Structure,
> #NVM Command Set.
> 
> 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. In addition,
> again unlike SCSI, there is no dedicated atomic write command - a write
> which adheres to the atomic size limit and boundary is implicitly atomic.
> 
> If NSFEAT bit 1 is set, the following parameters are of interest:
> - NAWUPF (Namespace Atomic Write Unit Power Fail)
> - NABSPF (Namespace Atomic Boundary Size Power Fail)
> - NABO (Namespace Atomic Boundary Offset)
> 
> and we set request_queue limits as follows:
> - atomic_write_unit_max = rounddown_pow_of_two(NAWUPF)
> - atomic_write_max_bytes = NAWUPF
> - atomic_write_boundary = NABSPF
> 
> If in the unlikely scenario that NABO is non-zero, then atomic writes will
> not be supported at all as dealing with this adds extra complexity. This
> policy may change in future.
> 
> In all cases, atomic_write_unit_min is set to the logical block size.
> 
> If NSFEAT bit 1 is unset, the following parameter is of interest:
> - AWUPF (Atomic Write Unit Power Fail)
> 
> and we set request_queue limits as follows:
> - atomic_write_unit_max = rounddown_pow_of_two(AWUPF)
> - atomic_write_max_bytes = AWUPF
> - atomic_write_boundary = 0
> 
> The block layer requires that the atomic_write_boundary value is a
> power-of-2. However, it is really only required that atomic_write_boundary
> be a multiple of atomic_write_unit_max. As such, if NABSPF were not a
> power-of-2, atomic_write_unit_max could be reduced such that it was
> divisible into NABSPF. However, this complexity will not be yet supported.
> 
> A helper function, nvme_valid_atomic_write(), is also added for the
> submission path to verify that a request has been submitted to the driver
> will actually be executed atomically.

Maybe patch 11 should be folded into this one. No bigged, the series as
a whole looks good.

Reviewed-by: Keith Busch <kbusch@xxxxxxxxxx>