[MOCKUP] x86/mm: Lightweight lazy mm refcounting

[Matthew Wilcox]

On Wed, Dec 02, 2020 at 09:25:51PM -0800, Andy Lutomirski wrote:
> This code compiles, but I haven't even tried to boot it.  The earlier
> part of the series isn't terribly interesting -- it's a handful of
> cleanups that remove all reads of ->active_mm from arch/x86.  I've
> been meaning to do that for a while, and now I did it.  But, with
> that done, I think we can move to a totally different lazy mm refcounting
> model.

I went back and read Documentation/vm/active_mm.rst recently.

I think it's useful to think about how this would have been handled if
we'd had RCU at the time.  Particularly:

Linus wrote:
> To support all that, the "struct mm_struct" now has two counters: a
> "mm_users" counter that is how many "real address space users" there are,
> and a "mm_count" counter that is the number of "lazy" users (ie anonymous
> users) plus one if there are any real users.

And this just makes me think RCU freeing of mm_struct.  I'm sure it's
more complicated than that (then, or now), but if an anonymous process
is borrowing a freed mm, and the mm is freed by RCU then it will not go
away until the task context switches.  When we context switch back to
the anon task, it'll borrow some other task's MM and won't even notice
that the MM it was using has gone away.