On Mon 12-08-19 10:56:20, Joel Fernandes wrote: > On Thu, Aug 08, 2019 at 10:00:44AM +0200, Michal Hocko wrote: > > On Wed 07-08-19 17:31:05, Joel Fernandes wrote: > > > On Wed, Aug 07, 2019 at 01:58:40PM -0700, Andrew Morton wrote: > > > > On Wed, 7 Aug 2019 16:45:30 -0400 Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > On Wed, Aug 07, 2019 at 01:04:02PM -0700, Andrew Morton wrote: > > > > > > On Wed, 7 Aug 2019 13:15:54 -0400 "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > In Android, we are using this for the heap profiler (heapprofd) which > > > > > > > profiles and pin points code paths which allocates and leaves memory > > > > > > > idle for long periods of time. This method solves the security issue > > > > > > > with userspace learning the PFN, and while at it is also shown to yield > > > > > > > better results than the pagemap lookup, the theory being that the window > > > > > > > where the address space can change is reduced by eliminating the > > > > > > > intermediate pagemap look up stage. In virtual address indexing, the > > > > > > > process's mmap_sem is held for the duration of the access. > > > > > > > > > > > > So is heapprofd a developer-only thing? Is heapprofd included in > > > > > > end-user android loads? If not then, again, wouldn't it be better to > > > > > > make the feature Kconfigurable so that Android developers can enable it > > > > > > during development then disable it for production kernels? > > > > > > > > > > Almost all of this code is already configurable with > > > > > CONFIG_IDLE_PAGE_TRACKING. If you disable it, then all of this code gets > > > > > disabled. > > > > > > > > > > Or are you referring to something else that needs to be made configurable? > > > > > > > > Yes - the 300+ lines of code which this patchset adds! > > > > > > > > The impacted people will be those who use the existing > > > > idle-page-tracking feature but who will not use the new feature. I > > > > guess we can assume this set is small... > > > > > > Yes, I think this set should be small. The code size increase of page_idle.o > > > is from ~1KB to ~2KB. Most of the extra space is consumed by > > > page_idle_proc_generic() function which this patch adds. I don't think adding > > > another CONFIG option to disable this while keeping existing > > > CONFIG_IDLE_PAGE_TRACKING enabled, is worthwhile but I am open to the > > > addition of such an option if anyone feels strongly about it. I believe that > > > once this patch is merged, most like this new interface being added is what > > > will be used more than the old interface (for some of the usecases) so it > > > makes sense to keep it alive with CONFIG_IDLE_PAGE_TRACKING. > > > > I would tend to agree with Joel here. The functionality falls into an > > existing IDLE_PAGE_TRACKING config option quite nicely. If there really > > are users who want to save some space and this is standing in the way > > then they can easily add a new config option with some justification so > > the savings are clear. Without that an additional config simply adds to > > the already existing configurability complexity and balkanization. > > Michal, Andrew, Minchan, > > Would you have any other review comments on the v5 series? This is just a new > interface that does not disrupt existing users of the older page-idle > tracking, so as such it is a safe change (as in, doesn't change existing > functionality except for the draining bug fix). I hope to find some more time to finish the review but let me point out that "it's new it is regression safe" is not really a great argument for a new user visible API. If the API is flawed then this is likely going to kick us later and will be hard to fix. I am still not convinced about the swap part of the thing TBH. -- Michal Hocko SUSE Labs
