Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Christoph Hellwig]

On Sat, Dec 08, 2018 at 10:09:26AM -0800, Dan Williams wrote:
> Another fix that may put pressure 'struct page' is resolving the
> untenable situation of dax being incompatible with reflink, i.e.
> reflink currently requires page-cache pages. Dave has talked about
> silently establishing page-cache entries when a dax-page is cow'd for
> reflink, but I wonder if we could go the other way and introduce the
> mechanism of a page belonging to multiple mappings simultaneously and
> managed by the filesystem.

FYI, I had a a prototype for DAX + reflink that didn't require
the page cache, although it badly reimplemented parts of it.  But
that was a long time ago, before we started requiring struct page
for the DAX memory.