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

[Dan Williams]

On Thu, Aug 31, 2017 at 8:04 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
>
> BTW. if the system is capable of flushing caches on power failure, it is
> system-wide feature, it is not per-device feature. The CPU caches may
> store data for any persistent memory device.
>
> You either have enough remaining power to flush the CPU caches or not.

Whether you need to flush caches or not can be a per-address range
attribute and can change over time.