[dm-devel] [PATCH] dax: remove the pmem_dax_ops->flush abstraction

[Dan Williams]

On Thu, Aug 31, 2017 at 7:49 PM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>
>
> On Thu, 31 Aug 2017, Dan Williams wrote:
>
>> On Thu, Aug 31, 2017 at 6:47 PM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>> > The patch abebfbe2f731 ("dm: add ->flush() dax operation support") is
>> > buggy. A dm device may be composed of multiple underlying devices and all
>> > of them need to be flushed. The patch just routes the flush request to the
>> > first device and ignores the other devices.
>> >
>> > It could be fixed by adding more complex logic to the device mapper. But
>> > there is only one implementation of the method pmem_dax_ops->flush - that
>> > is pmem_dax_flush() - and it calls arch_wb_cache_pmem(). Consequently, we
>> > don't need the pmem_dax_ops->flush abstraction at all, we can call
>> > arch_wb_cache_pmem() directly from dax_flush() because dax_dev->ops->flush
>> > couldn't ever reach anything different from arch_wb_cache_pmem().
>>
>> Unfortunately, this is not true, see usage of DAXDEV_WRITE_CACHE. This
>> is for platforms that arrange for cpu caches to be flushed on
>> power-fail, like standalone storage appliances, where it would be a
>> waste for the kernel to track and flush dirty cachelines for dax.
>> Theoretically this could be done on a per-address range basis (think
>> battery backing a subset of the system memory). I think we need to fix
>> the routing to flush the same dax_device that ->direct_access() was
>> invoked.
>
> With my patch, dax_flush checks DAXDEV_WRITE_CACHE and doesn't do anything
> if it's not set - it just doesn't go through device mapper.

True, but there's no guarantee that arch_wb_pmem() will be sufficient
going forward and the reason we routed this to the driver in the first
place was to allow driver / platform specific flush implementations.

>
>> > It should be also pointed out that for some uses of persistent memory it
>> > is needed to flush only very small amount of data (such as 1 cacheline),
>> > and it would be overkill if we go through that device mapper machinery for
>> > a single flushed cache line.
>>
>> I think this is more an argument to not enable DAX on that
>> device-mapper topology if operation routing impacts performance. DAX
>> is meant to get the kernel out of the way most of the time.
>
> I don't understand what do you mean? I am writing a device mapper target
> that uses persistent memory as a cache. When flushing the metadata,
> sometimes it is needed to flush single cache lines. Ideally, I would use
> the clwb instruction for this purpose, but that clwb is hidden behind a
> complicated pmem_dax_ops->flush abstraction (and that abstraction will get
> even more complicated if implemented correctly). How do you think I should
> write that target without DAX?

I don't think you want DAX for this, I think you want direct control
of a persistent-memory address range.  For example the brd driver
supports DAX but I wouldn't recommend using it as the basis for a
directly managed cache.

So I think we need to dive deeper into what an in kernel interface to
access a contiguous persistent memory range looks like. DAX was not
designed with that use case in mind.