Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5

[Christoph Hellwig]

On Mon, Mar 16, 2020 at 10:52:24AM +0100, Jan Kara wrote:
> > This sounds reasonable to me.
> > 
> > As for deprecating the mount option, I think at a minimum it needs to
> > continue be accepted as an option even if it is ignored to not break
> > existing setups.
> 
> Agreed. But that's how we usually deprecate mount options. Also I'd say
> that statx() support for reporting DAX state and some education of
> programmers using DAX is required before we deprecate the mount option
> since currently applications check 'dax' mount option to determine how much
> memory they need to set aside for page cache before they consume everything
> else on the machine...

I don't even think we should deprecate it.  It isn't painful to maintain
and actually useful for testing.  Instead we should expand it into a
tristate:

  dax=off
  dax=flag
  dax=always

where the existing "dax" option maps to "dax=always" and nodax maps
to "dax=off". and dax=flag becomes the default for DAX capable devices.