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

[Joel Fernandes]

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. 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.

This is just an effort to make the idle tracking a little bit better. We
would like to not lose the 'accessed' information of the pages.

Initially, I had proposed what you are suggesting as well however the above
reasons made me to do it like this. Also Minchan and Konstantin suggested
this, so there are more people interested in the swap idle bit. Minchan, can
you provide more thoughts here? (He is on 2-week vacation from today so
hopefully replies before he vanishes ;-)).

Also assuming all swap pages as idle has other "semantic" issues. It is quite
odd if a swapped page is automatically marked as idle without userspace
telling it to. Consider the following set of events: 1. Userspace marks only
a certain memory region as idle. 2. Userspace reads back the bits
corresponding to a bigger region. Part of this bigger region is swapped.
Userspace expects all of the pages it did not mark, to have idle bit set to
'0' because it never marked them as idle. However if it is now surprised by
what it read back (not all '0' read back). Since a page is swapped, it will
be now marked "automatically" as idle as per your proposal, even if userspace
never marked it explicity before. This would be quite confusing/ambiguous.

I will include this and other information in future commit messages.

thanks,

 - Joel