Re: [PATCH v4 3/5] [RFC] arm64: Add support for idle bit in swap PTE

Minchan Kim
2019-08-06T11:07:49Z

On Tue, Aug 06, 2019 at 12:47:55PM +0200, Michal Hocko wrote:
> On Tue 06-08-19 06:36:27, Joel Fernandes wrote:
> > On Tue, Aug 06, 2019 at 10:42:03AM +0200, Michal Hocko wrote:
> > > On Mon 05-08-19 13:04:49, Joel Fernandes (Google) wrote:
> > > > This bit will be used by idle page tracking code to correctly identify
> > > > if a page that was swapped out was idle before it got swapped out.
> > > > Without this PTE bit, we lose information about if a page is idle or not
> > > > since the page frame gets unmapped.
> > >
> > > And why do we need that? Why cannot we simply assume all swapped out
> > > pages to be idle? They were certainly idle enough to be reclaimed,
> > > right? Or what does idle actualy mean here?
> >
> > Yes, but other than swapping, in Android a page can be forced to be swapped
> > out as well using the new hints that Minchan is adding?
>
> Yes and that is effectivelly making them idle, no?

1. mark page-A idle which was present at that time.
2. run workload
3. page-A is touched several times
4. *sudden* memory pressure happen so finally page A is finally swapped out
5. now see the page A idle - but it's incorrect.