Re: [PATCH v2 2/2] libnvdimm, region: sysfs trigger for nvdimm_flush()

[Ross Zwisler]

On Mon, Apr 24, 2017 at 04:50:01PM -0700, Dan Williams wrote:
> The nvdimm_flush() mechanism helps to reduce the impact of an ADR
> (asynchronous-dimm-refresh) failure. The ADR mechanism handles flushing
> platform WPQ (write-pending-queue) buffers when power is removed. The
> nvdimm_flush() mechanism performs that same function on-demand.
> 
> When a pmem namespace is associated with a block device, an
> nvdimm_flush() is triggered with every block-layer REQ_FUA, or REQ_FLUSH
> request. These requests are typically associated with filesystem
> metadata updates. However, when a namespace is in device-dax mode,
> userspace (think database metadata) needs another path to perform the
> same flushing. In other words this is not required to make data
> persistent, but in the case of metadata it allows for a smaller failure
> domain in the unlikely event of an ADR failure.
> 
> The new 'flush' attribute is visible when the individual DIMMs backing a
> given interleave-set are described by platform firmware. In ACPI terms
> this is "NVDIMM Region Mapping Structures" and associated "Flush Hint
> Address Structures". Reads return "1" if the region supports triggering
> WPQ flushes on all DIMMs. Reads return "0" the flush operation is a
> platform nop, and in that case the attribute is read-only.
> 
> Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
> Cc: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  drivers/nvdimm/region_devs.c |   41 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> index 24abceda986a..c48f3eddce2d 100644
> --- a/drivers/nvdimm/region_devs.c
> +++ b/drivers/nvdimm/region_devs.c
> @@ -255,6 +255,35 @@ static ssize_t size_show(struct device *dev,
>  }
>  static DEVICE_ATTR_RO(size);
>  
> +static ssize_t flush_show(struct device *dev,
> +		struct device_attribute *attr, char *buf)
> +{
> +	struct nd_region *nd_region = to_nd_region(dev);
> +
> +	/*
> +	 * NOTE: in the nvdimm_has_flush() error case this attribute is
> +	 * not visible.
> +	 */
> +	return sprintf(buf, "%d\n", nvdimm_has_flush(nd_region));
> +}
> +
> +static ssize_t flush_store(struct device *dev, struct device_attribute *attr,
> +		const char *buf, size_t len)
> +{
> +	bool flush;
> +	int rc = strtobool(buf, &flush);
> +	struct nd_region *nd_region = to_nd_region(dev);
> +
> +	if (rc)
> +		return rc;
> +	if (!flush)
> +		return -EINVAL;

Is there a benefit to verifying whether the user actually pushed a "1" into
our flush sysfs entry?  Why have an -EINVAL error case at all?

Flushing is non-destructive and we don't actually need the user to give us any
data, so it seems simpler to just have this code flush, regardless of what
input we received.

> +	nvdimm_flush(nd_region);
> +
> +	return len;
> +}
> +static DEVICE_ATTR_RW(flush);