Re: [PATCH v4 1/5] mm/page_idle: Add per-pid idle page tracking using virtual indexing

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Aug 06, 2019 at 10:56:05AM +0200, Michal Hocko wrote:
> On Mon 05-08-19 13:04:47, Joel Fernandes (Google) wrote:
> > The page_idle tracking feature currently requires looking up the pagemap
> > for a process followed by interacting with /sys/kernel/mm/page_idle.
> > Looking up PFN from pagemap in Android devices is not supported by
> > unprivileged process and requires SYS_ADMIN and gives 0 for the PFN.
> > 
> > This patch adds support to directly interact with page_idle tracking at
> > the PID level by introducing a /proc/<pid>/page_idle file.  It follows
> > the exact same semantics as the global /sys/kernel/mm/page_idle, but now
> > looking up PFN through pagemap is not needed since the interface uses
> > virtual frame numbers, and at the same time also does not require
> > SYS_ADMIN.
> > 
> > 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.
> 
> As already mentioned in one of the previous versions. The interface
> seems sane and the usecase as well. So I do not really have high level
> objections.

That is great to know.

> From a quick look at the patch I would just object to pulling swap idle
> tracking into this patch because it makes the review harder and it is
> essentially a dead code until a later patch. I am also not sure whether
> that is really necessary and it really begs for an explicit
> justification.

Ok I will split it out, and also expand on the need for it a bit more.

> 
> I will try to go through the patch more carefully later as time allows.

Thanks a lot.

> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> -- 
> Michal Hocko
> SUSE Labs

 - Joel