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

[Dan Williams]

On Thu, Apr 27, 2017 at 12:17 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Thu, Apr 27, 2017 at 11:41 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
>> Dan Williams <dan.j.williams@xxxxxxxxx> writes:
>>
>>>> The sentiment is that programs shouldn't have to grovel around in sysfs
>>>> to do stuff related to an open file descriptor or mapping.  I don't take
>>>> issue with the name.  I do worry that something like 'wpq_drain' may be
>>>> too platform specific, though.  The NVM Programming Model specification
>>>> is going to call this "deep flush", so maybe that will give you
>>>> some inspiration if you do want to change the name.
>>>
>>> I'll change to "deep_flush", and I quibble that this is related to a
>>> single open file descriptor or mapping. It really is a "region flush"
>>> for giving extra protection for global metadata, but the persistence
>>> of individual fds or mappings is handled by ADR. I think an ioctl
>>> might give the false impression that every time you flush a cacheline
>>> to persistence you need to call the ioctl.
>>
>> fsync, for example, may affect more than one fd--all data in the drive
>> write cache will be flushed.  I don't see how this is so different.  I
>> think a sysfs file is awkward because it requires an application to
>> chase down the correct file in the sysfs hierarchy.  If the application
>> already has an open fd or a mapping, it should be able to operate on
>> that.
>
> I'm teetering, but still leaning towards sysfs. The use case that
> needs this is device-dax because we otherwise silently do this behind
> the application's back on filesystem-dax for fsync / msync. A
> device-dax ioctl would be straightforward, but 'deep flush' assumes
> that the device-dax instance is fronting persistent memory. There's
> nothing persistent memory specific about device-dax except that today
> only the nvdimm sub-system knows how to create them, but there's
> nothing that prevents other memory regions from being mapped this way.
> So I'd rather this persistent memory specific mechanism stay with the
> persistent memory specific portion of the interface rather than plumb
> persistent memory details out through the generic device-dax interface
> since we have no other intercept point like we do in the
> filesystem-dax case to hide this flush.

We also still seem to need a discovery mechanism as I've had questions
about "how do I tell if my system supports deep flush?". That's where
sysfs is much better than an ioctl. The need for deep flush discovery
tips the scales, at least for me, to also do deep flush triggering
through the same interface.