Re: [PATCH v8 1/1] f2fs: dax: implement direct access

[Christoph Hellwig]

On Wed, Jul 26, 2017 at 10:11:08AM -0700, Dan Williams wrote:
> Until HMAT came along we had no data in the kernel how to pick a sane
> default, but we could now very easily make a "if pmem performance <
> dram, disable dax by default" policy in the kernel.

I'd rather do it the other way around - if HMAT is present and
pmem performance >= dram use dax.  Else require the explicit -o dax
for now to enable it.  If an explicit -o nodax is specified disable
DAX even if HMAT says it is faster.

> The question for this patch is do we want to add yet another
> filesystem that adds "-o dax" or require use of per-inode flags to
> enable dax.

Please stick to the mount option.  After spending a lot of time with
DAX and various memory techologies I'm pretty confident that the inode
flag is the wrong thing to do.