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
