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

On Tue, Aug 06, 2019 at 01:14:52PM +0200, Michal Hocko wrote:

On Tue 06-08-19 20:07:37, Minchan Kim wrote:

quoted

On Tue, Aug 06, 2019 at 12:47:55PM +0200, Michal Hocko wrote:

quoted

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

quoted

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?

1. mark page-A idle which was present at that time.
2. run workload
3. page-A is touched several times
4. *sudden* memory pressure happen so finally page A is finally swapped out
5. now see the page A idle - but it's incorrect.

Could you expand on what you mean by idle exactly? Why pageout doesn't
really qualify as "mark-idle and reclaim"? Also could you describe a
usecase where the swapout distinction really matters and it would lead
to incorrect behavior?

Michal,
Did you read this post ? :
https://lore.kernel.org/lkml/20190806104715.GC218260@google.com/T/#m4ece68ceaf6e54d4d29e974f5f4c1080e733f6c1 (local)

Just wanted to be sure you did not miss it.

thanks,

 - Joel