Re: Reference count on pages held in secondary MMUs

[Christoffer Dall <christoffer.dall@arm.com>]

On Sun, Jun 09, 2019 at 11:37:19AM +0200, Paolo Bonzini wrote:
> On 09/06/19 10:18, Christoffer Dall wrote:
> > In some sense, we are thus maintaining a 'hidden', or internal,
> > reference to the page, which is not counted anywhere.
> > 
> > I am wondering if it would be equally valid to take a reference on the
> > page, and remove that reference when unmapping via MMU notifiers, and if
> > so, if there would be any advantages/drawbacks in doing so?
> 
> If I understand correctly, I think the MMU notifier would not fire if
> you took an actual reference; the page would be pinned in memory and
> could not be swapped out.
> 

That was my understanding too, but I can't find the code path that would
support this theory.

The closest thing I could find was is_page_cache_freeable(), and as far
as I'm able to understand that code, that is called (via pageout()) later in
shrink_page_list() than try_to_unmap() which fires the MMU notifiers
through the rmap code.

It is entirely possible that I'm looking at the wrong place and missing
something overall though?


Thanks,

    Christoffer