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

[Minchan Kim <minchan@kernel.org>]

On Tue, Aug 06, 2019 at 01:57:03PM +0200, Michal Hocko wrote:
> On Tue 06-08-19 07:14:46, Joel Fernandes 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?
> > 
> > That depends on how you think of it.
> 
> I would much prefer to have it documented so that I do not have to guess ;)
> 
> > If you are thinking of a monitoring
> > process like a heap profiler, then from the heap profiler's (that only cares
> > about the process it is monitoring) perspective it will look extremely odd if
> > pages that are recently accessed by the process appear to be idle which would
> > falsely look like those processes are leaking memory. The reality being,
> > Android forced those pages into swap because of other reasons. I would like
> > for the swapping mechanism, whether forced swapping or memory reclaim, not to
> > interfere with the idle detection.
> 
> Hmm, but how are you going to handle situation when the page is unmapped
> and refaulted again (e.g. a normal reclaim of a pagecache)? You are
> losing that information same was as in the swapout case, no? Or am I
> missing something?

If page is unmapped, it's not a idle memory any longer because it's
free memory. We could detect the pte is not present.

If page is refaulted, it's not a idle memory any longer because it's
accessed again. We could detect it because the newly allocated page
doesn't have a PG_idle page flag.

Both case, idle page tracking couldn't report them as IDLE so it's okay.