Re: [PATCH v2] libnvdimm/region: Update nvdimm_has_flush() to handle ND_REGION_ASYNC

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 4/6/21 5:07 PM, Vaibhav Jain wrote:
> Hi Aneesh,
> Thanks for looking into this patch.
> 
> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
> 
>> Vaibhav Jain <vaibhav@linux.ibm.com> writes:
>>
>>> In case a platform doesn't provide explicit flush-hints but provides an
>>> explicit flush callback via ND_REGION_ASYNC region flag, then
>>> nvdimm_has_flush() still returns '0' indicating that writes do not
>>> require flushing. This happens on PPC64 with patch at [1] applied,
>>> where 'deep_flush' of a region was denied even though an explicit
>>> flush function was provided.
>>>
>>> Similar problem is also seen with virtio-pmem where the 'deep_flush'
>>> sysfs attribute is not visible as in absence of any registered nvdimm,
>>> 'nd_region->ndr_mappings == 0'.
>>>
>>> Fix this by updating nvdimm_has_flush() adding a condition to
>>> nvdimm_has_flush() testing for ND_REGION_ASYNC flag on the region
>>> and see if a 'region->flush' callback is assigned. Also remove
>>> explicit test for 'nd_region->ndr_mapping' since regions can be marked
>>> 'ND_REGION_SYNC' without any explicit mappings as in case of
>>> virtio-pmem.
>>
>> Do we need to check for ND_REGION_ASYNC? What if the backend wants to
>> provide a synchronous dax region but with different deep flush semantic
>> than writing to wpq flush address?
>> ie,
> 
> For a synchronous dax region, writes arent expected to require any
> flushing (or deep-flush) so this function should ideally return '0' in
> such a case. Hence I had added the test for ND_REGION_ASYNC region flag.
> 

that is not correct. For example, we could ideally move the wpq flush as 
an nd_region->flush callback for acpi or we could implement a deep flush 
for a synchronous dax region exposed by papr_scm driver that ensures 
stores indeed reached the media managed by the hypervisor.

-aneesh
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