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

[Vaibhav Jain <vaibhav@linux.ibm.com>]

"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:

> 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.
Sure, makes sense now. I have sent out a v3 of this patch with this
addressed.

>
> -aneesh

-- 
Cheers
~ Vaibhav
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