Re: [PATCH v5 1/6] mm/page_idle: Add per-pid idle page tracking using virtual index
From: Michal Hocko
Date: Thu Aug 08 2019 - 04:13:54 EST
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.