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

[Dan Williams]

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.


> 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.