On Tue, Jul 02, 2024 at 08:19:01PM +1000, Alistair Popple wrote:

quoted

(B) As long as we have subpage mapcounts, this prevents vmemmap
    optimizations [1]. Is that only used for device-dax for now and are
    there no plans to make use of that for fs-dax?

I don't have any plans to. This is purely focussed on refcounting pages
"like normal" so we can get rid of all the DAX special casing.

quoted

(C) We managed without so far :)

Indeed, although Christoph has asked repeatedly ([1], [2] and likely
others) that this gets fixed and I finally got sick of it coming up
everytime I need to touch something with ZONE_DEVICE pages :)

Also it removes the need for people to understand the special DAX page
recounting scheme and ends up removing a bunch of cruft as a bonus:

 59 files changed, 485 insertions(+), 869 deletions(-)

And that's before I clean up all the pgmap reference handling. It also
removes the pXX_trans_huge and pXX_leaf distinction. So we managed, but
things could be better IMHO.

Yes.  I can't wait for this series making the finish line.  There might
be more chance for cleanups and optimizations around ZONE_DEVICE, but
this alone is a huge step forward.