Re: [PATCH 00/14] libnvdimm: support sub-divisions of pmem for 4.9

[Dan Williams]

On Fri, Oct 7, 2016 at 11:19 AM, Linda Knippers <linda.knippers@xxxxxxx> wrote:
> Hi Dan,
>
> A couple of general questions...
>
> On 10/7/2016 12:38 PM, Dan Williams wrote:
>> With the arrival of the device-dax facility in 4.7 a pmem namespace can
>> now be configured into a total of four distinct modes: 'raw', 'sector',
>> 'memory', and 'dax'. Where raw, sector, and memory are block device
>> modes and dax supports the device-dax character device. With that degree
>> of freedom in the use cases it is overly restrictive to continue the
>> current limit of only one pmem namespace per-region, or "interleave-set"
>> in ACPI 6+ terminology.
>
> If I understand correctly, at least some of the restrictions were
> part of the Intel NVDIMM Namespace spec rather than ACPI/NFIT restrictions.
> The most recent namespace spec on pmem.io hasn't been updated to remove
> those restrictions.  Is there a different public spec?

Yes, this is Linux specific and use of this capability needs to be
cognizant that it could create a configuration that is not understood
by EFI, or other OSes (including older Linux implementations).  I plan
to add documentation to ndctl along these lines.  This is similar to
the current situation with 'pfn' and 'dax' info blocks that are also
Linux specific.  However, I should note that this implementation
changes none of the interpretation of the fields nor layout of the
existing label specification.  It simply allows two pmem labels that
happen to appear in the same region to result in two namespaces rather
than 0.

>> This series adds support for reading and writing configurations that
>> describe multiple pmem allocations within a region.  The new rules for
>> allocating / validating the available capacity when blk and pmem regions
>> alias are (quoting space_valid()):
>>
>>    BLK-space is valid as long as it does not precede a PMEM
>>    allocation in a given region. PMEM-space must be contiguous
>>    and adjacent to an existing existing allocation (if one
>>    exists).
>
> Why is this new rule necessary?  Is this a HW-specific rule or something
> related to how Linux could possibly support something?  Why do we care
> whether blk-space is before or after pmem-space? If it's a HW-specific
> rule, then shouldn't the enforcement be in the management tool that
> configures the namespaces?

It is not HW specific, and it's not new in the sense that we already
arrange for pmem to be allocated from low addresses and blk to be
allocated from high addresses.  If another implementation violated
this constraint Linux would parse it just fine. The constraint is a
Linux decision to maximize available pmem capacity when blk and pmem
alias.  So this is a situation where Linux is liberal in what it will
accept when reading labels, but conservative on the configurations it
will create when writing labels.

>> Where "adjacent" allocations grow an existing namespace.  Note that
>> growing a namespace is potentially destructive if free space is consumed
>> from a location preceding the current allocation.  There is no support
>> for dis-continuity within a given namespace allocation.
>
> Are you talking about DPAs here?

No, this is referring to system-physical-address partitioning.

>> Previously, since there was only one namespace per-region, the resulting
>> pmem device would be named after the region.  Now, subsequent namespaces
>> after the first are named with the region index and a
>> ".<namespace-index>" suffix. For example:
>>
>>       /dev/pmem0.1
>
> According to the existing namespace spec, you can already have multiple
> block namespaces on a device. I've not see a system with block namespaces
> so what do those /dev entries look like?  (The dots are somewhat unattractive.)

Block namespaces result in devices with names like "/dev/ndblk0.0"
where the X.Y numbers are <region-index>.<namespace-index>.  This new
naming for pmem devices is following that precedent.  The "dot" was
originally adopted from Linux USB device naming.