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 ;) > > > 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? If page is unmapped, it's not a idle memory any longer because it's free memory. We could detect the pte is not present. If page is refaulted, it's not a idle memory any longer because it's accessed again. We could detect it because the newly allocated page doesn't have a PG_idle page flag. Both case, idle page tracking couldn't report them as IDLE so it's okay.
