Re: [PATCH v3] libnvdimm/region: Update nvdimm_has_flush() to handle explicit 'flush' callbacks

[Pankaj Gupta <pankaj.gupta.linux@gmail.com>]

Hi Vaibhav,

> In case a platform doesn't provide explicit flush-hints but provides an
> explicit flush callback, 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'.

In case of async flush callback, do we still need "deep_flush" ?

Thanks,
Pankaj
>
> Fix this by updating nvdimm_has_flush() adding a condition to
> nvdimm_has_flush() to test if a 'region->flush' callback is
> assigned. Also remove explicit test for 'nd_region->ndr_mapping' since
> regions may need 'flush' without any explicit mappings as in case of
> virtio-pmem.
>
> References:
> [1] "powerpc/papr_scm: Implement support for H_SCM_FLUSH hcall"
> https://lore.kernel.org/linux-nvdimm/161703936121.36.7260632399582101498.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:
>
> v3:
> * Removed the test for ND_REGION_SYNC to handle case where a
>   synchronous region still wants to expose a deep-flush function.
>   [ Aneesh ]
> * Updated patch title and description from previous patch
>   https://lore.kernel.org/linux-nvdimm/5e64778d-bf48-9f10-7d3d-5e530e5db590@linux.ibm.com
>
> 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..c4b17bdd527f 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 (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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