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

[Michal Hocko]

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?

> Also, even if they were idle enough to be swapped, there is a chance that they
> were marked as idle and *accessed* before the swapping. Due to swapping, the
> "page was accessed since we last marked it as idle" information is lost. I am
> able to verify this.
> 
> Idle in this context means the same thing as in page idle tracking terms, the
> page was not accessed by userspace since we last marked it as idle (using
> /proc/<pid>/page_idle).

Please describe a usecase and why that information might be useful.

-- 
Michal Hocko
SUSE Labs