[PATCH v3] libnvdimm, region: sysfs trigger for nvdimm_flush()

[Dan Williams]

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>
---
Changes in v3:
* Fixed up the permissions in the read-only case to 0444 instead of
  0400 as is typical for read-only sysfs attributes.

 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..9c4dc8bc759a 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;
+	nvdimm_flush(nd_region);
+
+	return len;
+}
+static DEVICE_ATTR_RW(flush);
+
 static ssize_t mappings_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
@@ -474,6 +503,7 @@ 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 +538,17 @@ 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) {
+		int has_flush = nvdimm_has_flush(nd_region);
+
+		if (has_flush == 1)
+			return a->mode;
+		else if (has_flush == 0)
+			return 0444;
+		else
+			return 0;
+	}
+
 	if (a != &dev_attr_set_cookie.attr
 			&& a != &dev_attr_available_size.attr)
 		return a->mode;