Re: [PATCH v5 2/6] mm/page_idle: Add support for handling swapped PG_Idle pages

[Joel Fernandes]

On Tue, Aug 13, 2019 at 05:04:50PM +0200, Michal Hocko wrote:
> On Wed 07-08-19 13:15:55, Joel Fernandes (Google) wrote:
> > Idle page tracking currently does not work well in the following
> > scenario:
> >  1. mark page-A idle which was present at that time.
> >  2. run workload
> >  3. page-A is not touched by workload
> >  4. *sudden* memory pressure happen so finally page A is finally swapped out
> >  5. now see the page A - it appears as if it was accessed (pte unmapped
> >     so idle bit not set in output) - but it's incorrect.
> > 
> > To fix this, we store the idle information into a new idle bit of the
> > swap PTE during swapping of anonymous pages.
> >
> > Also in the future, madvise extensions will allow a system process
> > manager (like Android's ActivityManager) to swap pages out of a process
> > that it knows will be cold. To an external process like a heap profiler
> > that is doing idle tracking on another process, this procedure will
> > interfere with the idle page tracking similar to the above steps.
> 
> This could be solved by checking the !present/swapped out pages
> right? Whoever decided to put the page out to the swap just made it
> idle effectively.  So the monitor can make some educated guess for
> tracking. If that is fundamentally not possible then please describe
> why.

But the monitoring process (profiler) does not have control over the 'whoever
made it effectively idle' process.

As you said it will be a guess, it will not be accurate.

I am curious what is your concern with using a bit in the swap PTE?

(Adding Konstantin as well since we may be interested in this, since we also
suggested this idea).

thanks,

 - Joel