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

[Linda Knippers]

On 04/21/2017 07:48 PM, 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. However, when a namespace is in device-dax mode, or namespaces
> are disabled, userspace needs another path.

Why would a user need to flush a disabled namespace?

> The new 'flush' attribute is visible when it can be determined that the
> interleave-set either does, or does not have DIMMs that expose WPQ-flush
> addresses, "flush-hints" in ACPI NFIT terminology. It returns "1" and
> flushes DIMMs, or returns "0" the flush operation is a platform nop.

It seems a little odd to me that reading a read-only attribute both
tells you that the device has flush hints and also triggers a flush.
This means that anyone at any time can cause a flush.  Do we want that?

-- ljk

> 
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  drivers/nvdimm/region_devs.c |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> index 8de5a04644a1..3495b4c23941 100644
> --- a/drivers/nvdimm/region_devs.c
> +++ b/drivers/nvdimm/region_devs.c
> @@ -255,6 +255,19 @@ 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);
> +
> +	if (nvdimm_has_flush(nd_region)) {
> +		nvdimm_flush(nd_region);
> +		return sprintf(buf, "1\n");
> +	}
> +	return sprintf(buf, "0\n");
> +}
> +static DEVICE_ATTR_RO(flush);
> +
>  static ssize_t mappings_show(struct device *dev,
>  		struct device_attribute *attr, char *buf)
>  {
> @@ -474,6 +487,7 @@ static DEVICE_ATTR_RO(resource);
>  
>  static struct attribute *nd_region_attributes[] = {
>  	&dev_attr_size.attr,
> +	&dev_attr_flush.attr,
>  	&dev_attr_nstype.attr,
>  	&dev_attr_mappings.attr,
>  	&dev_attr_btt_seed.attr,
> @@ -508,6 +522,9 @@ static umode_t region_visible(struct kobject *kobj, struct attribute *a, int n)
>  	if (!is_nd_pmem(dev) && a == &dev_attr_resource.attr)
>  		return 0;
>  
> +	if (a == &dev_attr_flush.attr && nvdimm_has_flush(nd_region) < 0)
> +		return 0;
> +
>  	if (a != &dev_attr_set_cookie.attr
>  			&& a != &dev_attr_available_size.attr)
>  		return a->mode;
> 
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