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

[Joel Fernandes <joel@joelfernandes.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 ;)

Sure :)

> > 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?

Yes you are right, it would have the same issue, thanks for bringing it up.
Should we rename this bit to PTE_IDLE and do the same thing that we are doing
for swap?

i.e. if (page_idle(page)) and page is a file page, then we write state
into the PTE of the page. Later on refault, the PTE bit would automatically
get cleared (just like it does on swap-in). But before refault, the idle
tracking code sees the page as still marked idle. Do you see any issue with that?


> > 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 ;-)).
> 
> We can move on with the rest of the series in the mean time but I would
> like to see a proper justification for the swap entries and why they
> should be handled special.

Ok, I will improve the changelog.


> > 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.
> 
> OK, I see. I guess the primary question I have is how do you distinguish
> Idle page which got unmapped and faulted in again from swapped out page
> and refaulted - including the time the pte is not present.

Ok, lets discuss more.

thanks Michal!

 - Joel