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

Michal Hocko
2019-08-06T11:14:57Z

On Tue 06-08-19 20:07:37, Minchan Kim wrote:
> 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.

Could you expand on what you mean by idle exactly? Why pageout doesn't
really qualify as "mark-idle and reclaim"? Also could you describe a
usecase where the swapout distinction really matters and it would lead
to incorrect behavior?

--
Michal Hocko
SUSE Labs