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

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

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,

>
> 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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