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

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.