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

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

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.

>
>>
>> References:
>> [1] "powerpc/papr_scm: Implement support for H_SCM_FLUSH hcall"
>> https://lore.kernel.org/linux-nvdimm/161703936121.36.7260632399	582101498.stgit@e1fbed493c87
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: c5d4355d10d4 ("libnvdimm: nd_region flush callback support")
>> Reported-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
>> Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
>> ---
>> Changelog:
>>
>> v2:
>> * Added the fixes tag and addressed the patch to stable tree [ Aneesh ]
>> * Updated patch description to address the virtio-pmem case.
>> * Removed test for 'nd_region->ndr_mappings' from beginning of
>>   nvdimm_has_flush() to handle the virtio-pmem case.
>> ---
>>  drivers/nvdimm/region_devs.c | 14 +++++++++-----
>>  1 file changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
>> index ef23119db574..cdf5eb6fa425 100644
>> --- a/drivers/nvdimm/region_devs.c
>> +++ b/drivers/nvdimm/region_devs.c
>> @@ -1234,11 +1234,15 @@ int nvdimm_has_flush(struct nd_region *nd_region)
>>  {
>>  	int i;
>>  
>> -	/* no nvdimm or pmem api == flushing capability unknown */
>> -	if (nd_region->ndr_mappings == 0
>> -			|| !IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API))
>> +	/* no pmem api == flushing capability unknown */
>> +	if (!IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API))
>>  		return -ENXIO;
>>  
>> +	/* Test if an explicit flush function is defined */
>> +	if (test_bit(ND_REGION_ASYNC, &nd_region->flags) && nd_region->flush)
>> +		return 1;
>> +
>
>
>         if (nd_region->flush)
>                 return 1
>
>
>> +	/* Test if any flush hints for the region are available */
>>  	for (i = 0; i < nd_region->ndr_mappings; i++) {
>>  		struct nd_mapping *nd_mapping = &nd_region->mapping[i];
>>  		struct nvdimm *nvdimm = nd_mapping->nvdimm;
>> @@ -1249,8 +1253,8 @@ int nvdimm_has_flush(struct nd_region *nd_region)
>>  	}
>>  
>>  	/*
>> -	 * The platform defines dimm devices without hints, assume
>> -	 * platform persistence mechanism like ADR
>> +	 * The platform defines dimm devices without hints nor explicit flush,
>> +	 * assume platform persistence mechanism like ADR
>>  	 */
>>  	return 0;
>>  }
>> -- 
>> 2.30.2
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-- 
Cheers
~ Vaibhav
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