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

[Dan Williams]

On Wed, Apr 26, 2017 at 1:38 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
> Dan Williams <dan.j.williams@xxxxxxxxx> writes:
>
>> 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.
>
> I can make peace with exposing this to userspace, though I am mostly
> against its use.  However, sysfs feels like the wrong interface.
> Believe it or not, I'd rather see this implemented as an ioctl.
>
> This isn't a NACK, it's me giving my opinion.  Do with it what you will.

I hate ioctls with a burning passion so I can't get on board with that
change, but perhaps the sentiment behind it is that this is too
visible and too attractive being called "flush" in sysfs? Would a name
more specific to the mechanism make it more palatable? Like
"flush_hint_trigger" or "wpq_drain"?