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

On Tue 06-08-19 06:36:27, Joel Fernandes wrote:

On Tue, Aug 06, 2019 at 10:42:03AM +0200, Michal Hocko wrote:

quoted

On Mon 05-08-19 13:04:49, Joel Fernandes (Google) wrote:

quoted

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